It was this post:
http://SimHQ.com/forum/ubbthreads.php/topics/4430961/re-wotr-pics#Post4430961And he made the distinction that he was referring to .303 guns (I did not confirm that, but I assume he's referring to some of the earlier, non-cannon fitted Brit planes). So it wasn't concerning .50 cal or 20mm cannons.
Again, I didn't spend any time to verify his concerns, but if .303s are ripping off wings, I'd have to agree.
By this time in warfare technology, 'rifle-caliber' air-to-air guns were becoming inadequate. They are (comparatively) weak guns, really only in service for rate of fire far superior to most cannons and a range that didn't suffer horrible trajectory drop.
Probably one of the best compromises was the very guns you mention: The good old .50 cal. Put 6 or 8 of those bad boys on anything, that's a lot of lead down range. And it can do reasonable damage.
But even then, as the war progressed, and by the time of Korea a few years later, everyone starts gravitating more toward cannons and, not long after that, AA missiles.