From what I learned Phantom Power came about as a way to keep every condenser mike from having to be plugged into a power supply. With phantom power your mixer, pre, whatever powers the mic.
Also, from what I have read, it does no harm to a mic that does NOT use phantom power to have it turned on, nor does it harm a mic that needs it to not have it. The mic that needs it just won't pickup very good... if at all. However, I do NOT know how a guitar or other input might react to phantom power being on. Best to be careful unless you know otherwise.
Mind you, I am not going to experiment!