Personally, I play with a laptop next to me, so I run Vent on that, and let the WoW sounds play on my desktop computer. But there are probably lots of people out there who use just one computer, so must alt-tab out to run Vent, and thus have their game sound and Vent sound coming through the same speakers. And there may even be a better way to do it -- maybe it would help things to have Vent coming out of a different speaker instead of through the headset, like from the surround sound speakers behind you.
In fact, it's too bad the in-game voice chat (remember that?) isn't a little more imaginative in how it plays the sound out -- it would be interesting to have players' voices positioned in 3D space around you, much like the game sounds already are. Vent is more or less necessary for raiding in big groups, but it's too bad that extra audio can sometimes break the illusion of the game.