I can easily look behind me while keeping the bike true on course, but that's not a reason to not use a mirror (at least in my experience of riding). My eyes are always darting around looking for things of concern, this includes behind me; I don't just check my six when I want to make a turn. Also, when I get into really heavy traffic, a repeated glance at the mirror is much more useful and safe than physically turning my head, when I'm getting ready for a turn.
I see some weird and sometimes scary things with what vehicles are doing in my mirror. I'm not totally trusting any driver and I want them to know (when they're doing stupid things) that I see them.