I did the same as the OP initially and only took Wrex and Ash in ME1 purely for firepower, but the last time I played it through I think I ended up taking Liara and Tali a lot more often, mainly because as a Soldier I could take care of damned near everything and it was more useful to have a biotic for when things got too much to handle and a tech expert so I could open doors/crates etc.
ME2, I never really had a fixed squad, I just took whoever I thought was appropriate. After a bit of familiarity with it, now I usually take whoever has the best/funniest dialogue. Just picked this up on Steam at Xmas with the Cerberus code, which I didn't have before (played my brother's copy and he forgot which e-mail address he registered the code to, and the other DLC was all registered to my e-mail) so I've got an excuse to have a play through all of this again.
In ME3, I usually took Garrus. They did a good job with his character in that game, he really did feel like an old friend you could count on. I usually teamed him with Liara or Ash, sometimes James if I wanted a change.