I like using mirrors when I'm working with weights to make sure my form and alignment are on target.
If I'm taking a class, mirrors are great to let me see the instructor because I'm too short to see over the other people in the group unless I'm right at the front.
When I'm doing cross fit, I have no interest in mirrors. The coach is always there to check form, and I'm so focused on gutting it out when I feel like I'm gonna die that I don't have the time or energy to spare myself a glance.
And I suspect I'd be horrified by the sweaty wreck looking back at me in the mirror if I did