Dear fellow tank,
Truth is, using the keyboard for skills and mouse for movement is the more efficient way. You can have very easy rotations set up with each finger at the ready.
HOWEVER
A great clicker will own an average keybinder anyday It's as simple as that. If you want to be the best, then yeah you have to be a keyboarder - but for everything lower than the best, clicking is just fine.
I can say this from experience, although not done endgame tankign in SWTOR, like yourself I've played all forms off tanking in WoW, both MT and OT. Never had a problem, infact without sounding arrogant - I was a pretty great tank And I've always used my mouse. I set my main moves in a nice rectangle (like 4x2) on the bottom bars, and I click away.
Personally I find it easier, a bit more engaging and more fun - so anyone who tells you that you can't be a good tank just cause your a clicker can well: look at your past experiences, they should speak for themselves.
I'd never suggest you leave or even argue with your guildies But I would have to question what kind of guild would ask you not to tank just because you click. Seems to me like they care too much for how you play - rather than how good you play.