For me,the mirror and its sight aperture allows a more accurate sighting on a terrain feature, cutting down on parallax error and variations due to shakiness in a hand held instrument... I am totally with you on carrying a dedicated signal mirror. in fact, I think the signal mirror is more critcal than the compass on most excursions.

For the trivial increase in weight, one then has two signal mirrors. If you want to attract attention, two mirrors flashing away should be really, really effective. I have never heard of this actually happening in real life. Perhaps others have? Aside from that, the capsule containing the mirror folds over and protects the mechanism (and just looks cool, to boot).

Another less common, but highly effective use of the compass. The first birthday present I gave my wife was a high end Suunto sighting compass, engraved on the side with "To Susan with love." It worked, as we just celebrated our 25th.....