I think it would be pretty damn awesome to see a band whose members really were different parts of a body and formed together on stage into a giant, hard-rockin', Voltron like robot. I would exchange my firstborn to see that.
But if I may compare band members to physiological and mental aspects of the musical body as a whole, I would say that:
The drums are its sense of balance. The guitar it's curiosity and sense of adventure. The bass as its instinct (yeah, and especially
that instinct.

). And the singer would be the conscience.
But I swear if your conscience starts to sing "When You Wish Upon a Star" you may have to put up a new wanted ad.