The problem with not having a mirror is you are less capable of taking accurate readings. When you use a non-mirror compass, the only way sight a bearing is to hold it up in front of you and view along the straight edged baseplate... but holding it in front of your eye so this is possible means you cannot see the needle to ensure it is still boxed in the meridian lines and also that it's still free-floating and not just being dragged by friction because your hand isn't perfectly level. I'd be surprised if you can level the compass, box the needle, and then bring it up to view without changing it less than a degree or so. The mirror lets you watch the needle WHILE sighting AND gives you a rifle sight through which you can sight a specific point.
It's one thing to orienteer (determine your position) you can put the compass on a flat surface, move it around on the map, get on your hands and knees to sight along the baseplate for triangulation, etc. I think anybody could do this with a simple baseplate type compass and some arithmetic (to account for declination). Navigation is the process of actually going somewhere and it's a whole different ballgame. It requires you to take readings dynamically and frequently. If you can't rapidly sight a bearing, you won't do it enough and you'll get lost OR you'll do it but will take twice as long to get anywhere. If you can't accurately sight on objects both near and far, your error will continue to compound itself and you'll be way off target, meaning you need to use a very large intentional error to avoid missing your target. Very large intentional error may mean you take far more time and travel much further because you couldn't head more directly to your target.
I can travel over a mile through dense hilly/rocky/forest and streams and come out with an accuracy of less than a half degree error from my starting point because I am able to sight with tight accuracy on objects that are relatively close to me (distant objects being blocked from view) and continue following my bearing with accuracy and precision. That is within 46ft over a mile of difficult travel (clambering over rocks, downed trees, around brushpiles, through streams, etc. How close can you get without a sighting mirror? On open ground with no trees (scottish higlands?) you can sight on something much further, thus reducing the number of chances for error, and get away with it, but then again, in terrain like that, you can see your destination from a mile away so it wouldn't matter. I totally understand what you mean, Geoff... many places the terrain is distinct enough that you don't need anything other than basic terrain knowledge to know where you are. The point is, when you NEED a compass to navigate, you need more than a baseplate.
Also, I don't see an inclinometer on this compass... Knowing the slope angle is a major factor in orienteering (when combined with altitude and/or aspect, you could narrow your location to just a couple possibilities). Seems a crime to omit the 2grams an inclinometer would cost when it provides a relatively useful bit of information.
So in the fervor that is lightweight backpacking, it seems to me efficiency is often overlooked. If you omit the extra three ounces that a sighting mirror costs but then have to travel greater distances (because of intentional error or because you get lost) with less certainty, what's the point? Masochism?