Reasons why old squad members will return:
In general, as many have said, ME2 was about building a team. Very much so. You had your team complete in ME1 after completing the first investigative mission, if you so chose. In ME2, you do not get your final member until two missions from the end (if you are trying for "perfect" ending). These people are not just run of the mill space adventurers. They are the best at what they do. How many new characters came around for Return of the Jedi? Admiral Ackbar and Nien Nunb...At least, those are the only two I can think of. And the ewoks. Point is, everyone important to the story, Luke, Han, Leia, Chewy, even Lando and Wedge, all were already involved. The story here was the people. You'd be hard pressed to find better people anywhere in the galaxy. Maybe there are a few hiding under a rock somewhere. I for one would still like to see Volus, Elcor, Batarians, and even a Hanar and Rachni working with Shepard (maybe not necessarily as squad members, but nevertheless on board). But as far as the other character types go, we've already got the cream of the crop. Sure, you could make newer, even better characters. But, if they are better, why didn't you just pick them up in the first place?
Specifically, there are good reasons most of the characters from both would stick around/rejoin.
Ashely/Kaiden: The survivor was always supposed to be important in ME3. The devs very often said this was why they were kept out of the party in 2. From a plot perspective, assuming Shepard leaves Cerberus there is no reason they would not return to help. Even if Shepard remains with TIM, the Reapers moving in gives pretty good motivation to help do something, while positioning to take Cerberus down and Shepard with it if need be afterwards.
Liara: So she's gotten into information brokering. Does this mean she has lost some operative skill? Doubt it. Information is one of the most dangerous businesses out there. In all likelihood the Shadow Broker will have some sort of major influence on the game. Her personal vendetta could become very important, in a positive or negative way.
Wrex: Wrex is perhaps the least likely of all characters to return. He's got serious business at home. Building a united Krogan army may be the best thing he can be doing right now. I'd love to have him back, but it might be better for him to not be with me so that at some point he can swoop in with his army and save my sorry butt from a battle going none too well.
Garrus: Garrus is one of the faces of Mass Effect. The events thus far have made him question the role of law enforcement and the laws themselves, abandon all which he formerly knew, and disfigured him. Yet he has remained strong. And he has remained loyal. He is, essentially, the war buddy/best friend type to many a Shepard. It makes no sense for him to go anywhere.
Tali: Tali has essentially given up any normal kind of life to travel with Shepard. She is just as loyal as Garrus, and whether you romance her or not she in some sense worships a good Shepard as a true hero. Some have said that perhaps she would travel back to the fleet and become a new Admiral. However, when it is brought up in game she is, at the least...Unenthusiastic about the idea. She would much rather be with Shepard, on the front line, than standing by the wayside awaiting his signal.
Grunt: Grunt is the ultimate warrior type. He will not rest until he has proven that he's the best there is. In what world does this mean anything other than traveling with Shepard straight down the Reapers' collective throat? At the end of the game, he mentions that if he and Shepard have defeated the most powerful force in the galaxy, that leaves them. Grunt has great respect for Shepard, and possibly somewhat of a rivalry as well. I can see him leaving.
Jacob: Jacob is an idealist. He is all about helping humanity and getting the job done. He very easily befriends Shepard and supports the mission without fear. He may be the least talented of all the squad mates, in terms of background skill, but he is still a veteran combatant and has no fear of what is to come. Being the idealist he is, he would never walk away from something this important.
Miranda: Miranda respects Shepard a great deal, and the fact that she picks Shepard over TIM when given the option speaks volumes. There is, again, no reason to speculate she would leave for any reason. As a second in command her only equal appears to be Garrus, which is just as important as her considerable combat prowess.
Mordin: Mordin may be getting old, but he might not be done just yet. ME3 Appears in all likelihood to begin immediately following ME2, or at least very soon after. The Professor can still have some tricks up his sleeve. At the least, I would say he will stick around as the Normandy's science officer, which I believe is a position which will be sticking around.
Samara/Morinth: Samara offers her friendship and help to a good Shepard for events after the battle with the Collectors. This seems a very likely reason to stay aboard. Some have suggested she may go back to being a Justicar...Justicating? Anyway, what would be for the greater good? tracking down some small-time criminals in Asari space or saving the galaxy? It seems a little shallow for her to help stop human colony abductions and then avoid the all-out battle for the sake of organic life itself. Morinth...may take some more convincing. But, having what is the Mass Effect equivalent of an Illithid Savant on your team would be well worth the trouble of trying everything to keep her around for a renegade Shepard.
Thane: Everyone seems to think Thane is going to die before or at the beginning of ME3. However, two things. Well, really three. Thane's disease is very much like tuberculosis, in that staying warm and dry makes the disease act more slowly. This can allow him to preserve himself longer should he so desire. Secondly, it would not actually be that hard to replace his affected tissues (lungs and airways) with cybernetics. These two options are only viable, however, considering that Shepard has given him a greater reason to live. If he does die, however, it would be very little surprise if his son does in fact replace him, sort of a way to keep the character alive without taking away the mortality that helped make Thane an interesting character.
Jack: Jack has got nowhere else to go. Sure, it was easy to get her to hate you, in which case she'll probably jump ship the first chance she gets. But, assuming not, she would not only be very valuable but also have a good reason to stay. If she trusts Shepard enough to not hate him, it would mean she has found someone who wasn't just trying to use her like all the others. And that is worth something. Worth fighting for.
Legion: Legion already wants to fight the Reapers. And they believe Shepard is the key to winning. There is not much chance they would leave, and in fact even if they die, would probably be replaced by an equal platform. Legion is the key to allying with the Geth, and can coordinate plans without even being there (as they can connect with the network through the ship). More importantly, their inclusion late in the game screams for more development, possibly louder than all others.
Zaeed: Sure, why not. Zaeed is a merc. This job will pay.Someone mentioned him dying beforehand of natural causes. According to the website he's only 40. Not seeing that happening. Certainly he was not terribly deep or well developed compared to the other characters, but he still has a great deal of potential and skill.
Of course, this all assumes surviving characters. But, there isn't really a reason to suggest that just because they can die they'll be sidelined. It's not like the work that goes into a given character will go to waste. The cost of voice acting is worth what it takes to build story. As I've said before, BG2 had 17 recruitable members, and any of them could be killed. They were not fully voiced, but they had far more dialogue. So, from a developing standpoint it is doable. For the story it seems necessary, at least for many characters. Anything less would be a cop out.