Other than 4e, which has seen very little play in my group, I'd say 3e had the lowest incidence of "non-core" classes. Very rarely did anyone want to mess with a PrC. When they did, it was as often one from the DMG as not. If you add Warlock and psionics to core, then there were only a handful of non-core classes over the life of 3e, in my group -- I played a 3.0 thief-acrobat, another player did a swashbuckler, a sword sage who lasted nearly to 3rd level, and we had an Order of the Bow Initiate dip for two levels (who would have been happier taking a feat to give melee archery). There may have been something else, but not so far as I recall.
2e had the highest non-core. As soon as the first kit was allowed, everyone realized that it was dumb to not take what amounted to free power. The result was a near 100% rate of something non-core.
1e was about 50%, with quite a few supplimental classes from Dragon or 3PP, as well as Unearthed Arcana. There were a couple of barbarians, a fistfull of acrobats, and notable alchemists, duelists, death masters, witches, archers, and even jesters with some others thrown in for flavor. I'd say the core classes were a good anchor, but non-core made up about 50% of characters/PC lifetime.
4e hasn't really been out long enough to tell, and my group is heading toward nWoD right now, but I suspect it would break down a lot like 1e did. The class model really seems to be similar in nature (favoring tailored classes over PrCs or kits). The ubiquity of Paragon Paths would be the big factor that would drive it up, depending on whether you consider them "classes" or not (and I would). So, I'd say 4e will be 50-90% non-core.