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If you are only carrying a compass as a backup,  the mirror is unnecessary.   It will just add weight and make the compass more fragile.

If you are actually interested in navigating using a compass then having one with a mirror is very helpful.

The sighting mirror makes it much easier to taking bearings and also generally gives you a clinometer feature allowing you to estimate the steepness of a slope.

The mirror also makes a useful vanity and emergency signalling mirror.

But the most useful feature to pay for is adjustable declination.  You do have to remember to set the declination correctly for your current location but once set it avoids you having to calculate the true bearing by manually adding or subtraction the error prone activity. 

Also when buying a compass make sure to get one for the the hemisphere you intend to use it in.  There are some more expensive models that work in both the northern and southern hemisphere but mostly they are designed to work in one or the other.

Another consideration is what maps you will be using since the grid on the plate of the compass generally comes in one of two varieties... USGS (1:24,000) with inch scales for the US and Metric (1:50,000) with mm scales for everywhere else.  Recreational maps are often slight odd scales so this may not matter as much to you .