The fashion police will be after you - they disapprove of anything not absolutley essential to make a bike go.
I got one when I first had a recumbent, 'cause it's harder to look behind you on them. Then I missed having one on my other bikes, so now most have them. It really depends where you ride how much use they are. I ride to work partly along a horrible 70mph trunk road. It's good to be able to see what's about to pass all the time without continually having to look round.
On the other hand, in town, you'll generally be actually turning and looking whether you have a mirror or not. You get a better view, and it also signals to following drivers that you might be human, and that you might be about to manouvre. (With a mirror, you can of course choose when and to whom to give those specific signals).
They vary a lot in how much you can see in them. If you want one, don't abandon the idea if the first one you try is a bit crap. There are probably better ones, or ones that suit your bike and your ride better.
If you tour with a spouse/partner/mate, a mirror is dead useful to see how far behind you've left them!
You have nothing to lose but your street cred (if you have any) from trying one. Just don't forget that "life saver" glance over your shoulder even when you have a mirror. It's called that for a good reason.