You need 1 MT but you do need someone that can take aggro and stay alive for a bit a couple of places.
1. Turret Breaks on EV
2. Puzzle EV
3. 2 Bounty Hunters Palace
4. Droid with Fire Palace
We are thinking of using a 2nd tank on Soa HM to have him break lighting balls.
This ^^^, though we often run two tank operations because we ended up with more tanks than we really need for the 8 person content but we're still running some people through the HM FPs. Bioware did not do a good job here seeing how class progression was going to work out. This could be dealt with by:
1) Changing the operations to legitimately need two tanks (traditional MMO approach).
2) Make respeccing less of a chore so that we can store a build even if we have to pay to switch between them.
3) Make dual spec part of the game.