That "comfort zone" thing can be nice but it robs you of the chance to become a more balanced musician. Every gig is a paid rehearsal and an opportunity to improve. Playing in the key of B (or somewhat more challenging: Db) or any key other than C gives you more experience in new fingering, scaling and soloing. It could make that grand piano look more inviting. I agree with getting enough songs under your belt to do the gig any way you have to but at that very instant is when you should add songs for the sake of stretching your abilities.

Same can be said for the play-by-ear-only crowd. I think any one-sided approach to performance is a hinderance to progress. I've known people who could only play by sheet music and what they lacked in improvisation skills they often made up for in sightreading ability and sheer precision. I wouldn't trade one set of skills for another but do my best to incorporate both.