Re: Ashley, Kaidan - They were set aside on purpose. I never implied, in any manner, that just because either of them could have died in ME1, they were bound to have minor roles in ME2. The fact that we can't get the remainder of the two (whoever you chose to spare) to join the squad, let alone killed in the Suicide Mission, means that he/she must stay alive for whatever comes is in ME3.
The latter also applies to Liara , but we can talk about her role in some other thread.
Tali's situation is unique. It's not that I don't want her to appear in ME3 at all. I just mean that her participation in the squad is bound to be minimal (unless done very early in the game). The knowledge that she carries is way too important for her to be galavanting across the galaxy blowing up Reapers. It's not something she can just download from her Omni Tool to someone else's database. I can think of no other character to continue (or trust with) her research. She has to be there if and when her people decide to take back their homeworld from the Geth.
And if something like the latter does take place, I don't want BioWare to simply condense it into a few lines of bland text, or worse yet - distract the player from all the important plot elements that have suddenly disappeared with a "grocery list" of loyalty missions that, albeit well designed, are utterly insignificant narrative garbage. [deletes rest of rant about the bad, lazy writing in ME2]
I suppose it could go in different directions, depending on whether the Quarians go to war with the Geth or not, whether or not the player chose exile for Tali, and not to mention the consequences of re-programming the Geth in that one station (via Legion's mission) should the player choose to do so. But anyways...
At any rate, as Mordin says, "Too much data. Too many variables," or something to that effect.