I can't for the life of me find the webpage I saw it on, but there's a company out there that designed and built a custom receiver *specifically* for use with AK mags, and mated it to a completed 7.62x39 upper. A couple of other boards I've read have had a few people who've tried these out, and they say that it doesn't seem to have the trouble of the AR-15's with the 7.62 uppers, most specifically how you don't need to use a special magazine.
Total cost for the whole rifle was just at $1,200, and this included a CAR style adjustable stock, and I believe the SAW style pistol grip. The upper that was shown was the flattop, so you could use it with optics or one of the optional carry handles. I want to say the upper was also manufactured specially for this rifle, but I may be mistaken and it's just a standard "conversion" upper.
As for why anyone would want one, I'll use myself as an example. I like the style and design of the AR-15, but I already own a few rifles that use 7.62x39, and therefor have a lot of ammo in that size already. IMHO, it would be pretty much the best of both worlds, having an AR that can use the same ammo as my SKS and AK's.