This is not about ability, I'm a new player in SWTOR, yes and hence all the newbie type questions.
Its about convenience. When a FP don't go well, I don't want to stick around arguing over petty little stuff or standing around feeling bad and waiting for the timer for me to leave. I want to drop group quickly, no hassle, no drama, and go on with my Class story. After all, a typical player will run the same FP multiple times and there're only so many choices you can make in the dialog (good/evil choices) before it gets boring. So I want to go into the FP, pew pew pew pew....if going is good great, if not, I want to leave quickly and do other stuff.
I'm still on my first toon so I'm not a level 50 toon sitting around with nothing to do/see. There are literally tons of content for me to do still and waiting around in a FP, though a few seconds I'm admit, seems a little pointless. Why not give us a instant leave group option just like WoW does?