The issue is how much Bioware wants to invest in VO that might very well not be used. Doing extensive dialogue trees for a character that may well be dead means less for one that is very much alive (money and time, and disk space, is finite, alas). However, Bioware actually have very good data on who died or survived the SM (from the in-game feedback), so they know which squad members were killed off the most. These are unlikely to get more than a cameo.
As Garrus and Tali are quite popular, it is likely most save states have them alive, and it would pay dividends with the fans to have at least a temporary mission, or even a sequence of them.
It seems Bioware are biting the bullet and having at least one character with major VO not being available, in the alternate VS. This makes sense, given the importance of the characters and their tiny role in ME2.
Also, this being the last game, they can kill characters off for good. Why should it be a numpty who gets offed in the opening 30 mins, and not Garrus? That would hurt, real good. Good motivation in that. Or Anderson. In fact, I'll call it. One of the main cast gets it in the opening sequence!
Also, Zaeed and Kasumi. As DLC, many players won't have had them in the first place to be killed in the SM, so there's possibly a larger chance of these 2 appearing than not. Kasumi, after all, was in possession of some dark secret that could very well alter the likelihood of other civs helping humanity.
Others could appear as recurring cameos, rather than one-offs. You could send other teams out to deal with some stuff, reporting back later.
Basically, there's loads of ways Bioware could do justice to former squadmates without having them as permanent companions.