The reliability of the AK mag vs all the problems posted online with running the 7.62x39 round in a AR15 type mag far outweighs the magazine capacity issue IMO.
We are now just seeing US manufacturers coming out with lowers that take the AK mags for the very reason I posted above. (MGI Hydra, CMMG Mutant, etc)
Robinson Arms XCR-L in 7.62x39 seems to feed well using those mags. There's a good vid that addresses this issue. Apparently some folks trying to run an AR upper in 7.62x39 using AR type mags don't have the right feed ramps in their upper to accommodate the thicker cartridge, or the proper gas system.
http://www.canadiangunnutz.com/forum/showthread.php/1220225-7-62x39-AR-Fix-Video