I would rather call Liara a Shepardmancer because she is actually the initiator most of the time. Also, she effectively "necroed" Shepard before ME2, so Shepardmancer is a fitting term here. 
Glad to hear Im not the only one who picked up on that. Not that it wasn't obvious, but you know. Lol. This was my point about why Liara is treated "as if she's my LI or something" in the goodbye scene. She has a rather strong connection with Shepard regardless of rather or not you're in a relationship with her.
1. lakus has a good point. The resources that would have been sunk here are not insignificant.
Just to clarify, in case there is some confusion... I totally agree with Iakus here. I never said anything to the contrary and my first post mentioned it though not as specifically. I was bit more general about it, but yeah. I'm not oblivious to the resources that would had to be thrown into this.
2. I disagree about some squad mates not having a good reason. In fact, I think the only one who has a really dumb reason to decline is Jacob (and one can argue that even that is in character
). Grunt's unit, even if wiped out is not gone completely and is being rebuilt. Also, in ME2, he finally gets to terms with being a Krogan and an Urdnot. It makes sense that he is no longer isolated from his peers, especially after he apparently impressively managed to get their respect between ME2 and 3.
I definitely agree with the sentiment and as I mentioned earlier I like that he's 'grown up'. I'm proud of my favorite tank-bred. I can understand that the company would be rebuilt but the entire squad was essentially killed* if you save the queen. I see that as a good opportunity to have Grunt return to the Normandy Krantt. Similar to how Garrus joins up in ME2 shortly after his entire squad was killed, even though its feasible that he could rebuild it.
Though I admit to being emotionally bias on this since, frankly, Grunt was one of my favorites and I always feel jaded that they considered James to be an adequate substitute.
*"Wave after wave of rachni overwhelmed the Aralakh Company on Utukku. While fighting an impressive enemy to the death is considered an honorable end for a krogan, Aralakh Company has effectively been wiped out."
Samara only swore herself to Shep for the ME2 mission. In fact, she explicitly states that after the SM is over, she will no longer be bound to Shep. While she has no particular reason not to join the Normandy crew again, it makes sense for a character, so routed in Asari society to fight with her people. Also, I realize a lot of people wanted to romance her but seriously, it would not have fit her sombre and distantly wise character at all IMO. I already think the Citadel scene is quite a stretch. It's another one of those instances where you have to weigh character development vs. fan service and BW made the right choices here IMO.
True, true. I concede that point. Still, it felt a bit lame to me. She is deeply routed in asari society, as you say, but she is a Justicar. They generally stick to asari space but that doesn't mean her code is bias to Asari. Unlike some other characters she is primarily a soldier. It's not like she can do more good as a politician, scientist or thief, fighting reaper troops is essentially what her job is (though not specific to the reaper faction, obviously). Her excuse was basically "my code tells me to fight where I am needed most". IMO, that is best served with Shepard. The battles Shepard fights are integral to the war effort and ultimately the defeat of the reapers.
During the goodbye on Earth she even mentions that its "about time I joined your battle". On this sentiment I definitely agree. She is fighting off reaper forces anyway so why not do so with Commander Shepard, the guy leading the charge and who is personally interwoven in every critical moment of the war. That seems like it would do more good, imo. Nearly every other character was doing something a bit more specific to their role in the war effort.
Jack was training and leading biotic students on the battlefield. Miranda was sabotaging Cerberus operations all over the place and tracking down her father which turned out to play a really big role for Cerberus. Kasumi is stealing critical key objects for the crucible project. Grunt was leading a company of skilled krogan. Wrex was leading all of Tuchunka. Mordin was curing the genophage. Much like Zaeed, Samara was just killing off reaper troops. This isn't a very specific role and is something that could had been achieved just as well with Shepard.
Sure Liara and the Virmire Survivor were barely in ME2 but that was only the second entry you know? ME3 is basically the last we will see of these characters so my favourites being shafted for the previous crew (even though they may have good reasons) and just having a few cameos was definitely annoying and sad
Sure, it was 'only' the second entry... but that doesn't change the fact that Liara is practically not present through the entire thing. At least you'll concede they had good reasons rather than calling it all illogical, implausible nonsense. That's a start. 
I would had liked more cameos, to be sure, but honestly, for the most part I was a bit surprised at how big SOME of their roles were. I believe Iakus said it best when he said they were doomed to be cameos from the start. So, personally, I was impressed with how much some of them were involved in the main story. Miranda and Thane, specifically.
I think everyone expected Mordin, Wrex and Legion to play important roles in the main story but I wasn't expecting Thane to come into the main story and biotically punch Mr. Space Ninja. That was pretty awesome. I also wasn't expecting Miranda to be so involved in the main plotline that leads to you tracking down the Illusive Man. I certainly wasn't expecting her faddah to have any role in it.
In short, it could had been much worse. What if all the characters were treated like Zaeed where its just one side mission where they make a very brief appearance, say a few lines then you practically never interact with them again without DLC. At least they wrote Miranda and Thane some part in the main story and gave them some relevance. Not as much as they should, given that they're former LI, but this could be said about practically all the LI going all the way back to the second game even.
I think we can also both agree that they should have joined us in the final battle like Jack with her students etc.
That is just one of the many things that I felt should had been done differently with that final battle. Though, I admit, it isn't high on my priority list of things they should had done. Not because I wouldn't want that but because there are bigger issues I have with the final mission then just that. In comparison, this is trivial. Lol.