That's a valid point, and my thinking was actually similar, even if I reached a different conclusion, to wit, a dynamic mic would be fine for bedroom demos of male rock vocals. In fact, I know rockers and rappers who get more than adequate results recording vox with these same mics. In addition, when you decide to get a mic specifically for recording vocals, you still have a multi-purpose mic that will last forever. Exactly because he could use it for live vox, or to mic an amp. The OP appeared to be considering a cheapo mic which he would quickly outgrow, and wouldn't be able to usefully re-purpose, and I wanted first of all to caution against that course of action.
It seems I'm explaining it much better now than I did then (~2yrs ago...).