I don't think I have ever gotten ANYTHING good from the wonder-trade, I have traded hidden abilities, lvl 50-60s and loads of starters including hoenn ones. And what do I get back? Dodgy named pidgies, Eevees, AND THE BLASTED 3 ELEMENTAL MONKIES! (I hate that they are so common in this game when I spent about an hour getting them in Black and white through shaking grass) I sometimes get decent Mawiles with good moves, but rarely. But hey, it gives me Pokemiles I can use to buy rare candies.
I know, some previously rare Mons are common now, while other Mons that you couldn't swing a stick without hitting in some places are rare. Take Woobat, for example. It took me forever to find one that didn't have Klutz, seeing as I had to depend on the Cave Shadow things.
Then there's Magikarp. This may well be the only game where a Gyarados is easier to find in the wild than a Magikarp. (Really! Magikarp are only found in certain bodies of water and you can only fish for them with the Old Rod, but once you upgrade to the Good Rod, Gyarados are everywhere. It gets annoying.) Maybe the reason is, even rookie players know what you can do with the "seemingly powerless" Magikarp at this point, and want to find one as soon as possible. (Especially since a Gyarados can murder your opponent in a Sky Battle.)