Perfectly true, the compass mirror is a makeshift, but it is way better than nothing. As Rafowell pointed out in an earlier post, an AMK mirror weighs about an ounce - not much of a weight penalty.

Thinking about the general question, I believe there is one environment where a signal mirror might be less effective - at sea or a similar body of water, at least for other vessels at your level. This is because it is fairly common at sea to experience "mirror flashes" from other vessels. With this in mind, aim several flashes in succession at an intended target - that would be unusual and should get attention.