TK's latest flight sim (the developer of EAW, by the way) is a 1960s era jet sim. Flyable are the F-4, Phantom F-104 Starfighter and others. The reason I bring it up is that it has mirrors on the cockpits, and they work really well.
A mirror WILL NOT save you from a bogey on your six that you didn't know about, because by the time you see it you're toast.
It does add a lot to immersion, and a sense of 3d space, however, since you see ground objects, or the horizon, or clouds and other things reflected in it. Especially in AH, when there are often periods where all you see from the forward view or 45 degree up view is blue sky, also seeing the location of the horizon in a reflection, or how quickly the landscape is scrolling past, would be helpful. In TK's Strike Fighters: Project 1, there are a lot of vapour trails so it's helpful for spotting those, too.
While there is a hit to framerates, it's not the old days where Voodoo1s were struggling to show the forward view with full details in GPL, and then enabling the mirrors made it a slide show. Video cards have buckets of processing power and texture memory compared to back then, and it could be implemented easily. Of course, they could be a user option.
FWIW, I liked the direct 6 view as looking up in the mirror in Spits in AW. While I didn't play AW much at all, that was one aspect that I really found kind of neat. They got around the frame rate hit by not rendering two different scenes on the same display (i.e. one couldn't see around the mirror, it only showed the reflection). It was a clever, low-tech application, that dealt with the "Linda Blair" dead-six view that many don't like.
Cheers,
phaetn