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OK. My .02
I like the mirror. It's useful as an accuracy aid with the compass - especially if you have known fixed points like summits or the like for orienting your map. It takes some practice, but the practice can be fun.
Then it's also useful for other things like checking your eye for foreign objects (and, I suppose, signaling, though I have never done that).
So, you don't HAVE to have one. I just like it.
One weird thing I found out the hard way , is that you can ruin a good compass by keeping it with your cell phone. True story: I actually ruined a nice Sylva this way. The needle just floats around without any conviction about magnetic north.