Probably becuase not all games support it and some that dont will crash. Hell the unreal engine for example if i try to run with out setting it to one cpu it crashs all the damn time. Wish they would fix that.
You're better off letting the OS split the load between winamp and WoW. It'll give better performance since they'll both share part of the two cores. Unless the game specifically requires it to be run on a single core, I recommend you leave that stuff alone.
Reason i wanted to specify is i thought maybe running WOW and winamp on different cores would make the game run better sense it isnt dual core game anywho...or is the comp smart enough to do that.
The OS should automatically load-balance across cores so they would end up running on different cores anyway, so leave it to the OS.
It was a good point earlier though about games that crash on multiple-core systems. Thats probably the only conventional reason to force affinity, unless you're running a server farm or something.
I can't imagine what the game code could be doing to cause it to require single core though. it must be relying on cache-misses or something. I'm an experienced software developer and couldn't even guess how you could write code to screw up like that even if you wanted to. Its probably more down to the compiler than the programmers code.