I'm truly split on this one. When commuting I usa a mirror. When traversing the high density neighborhoods I live and work in I find it a great tool, particularly at intersections. The din of noise is such that hearing just isn't that reliable and sometimes I want to peek without telegraphing it to the world.
Just another arrow in my quill, if you will.
On my roadie I don't, though sometimes I think I should. There I pick the roads and ride a bit faster and am able to be a bit more dynamic with my head movements. Less trafficed roads also make the doppler effect more useful for judging approaching vehicles.
Different circumstances call for different equipment.