On-board sound has gotten loads more better now than it was years ago. Most can do 24-bit@96KHz (probably with restrictions), have basic THX/Dolby support, an arrangement of effects, and can handle many channels. The only reason why anyone would get a sound card aside from audiophiles seeking better quality was gamers who wanted to take advantage of Creative's offerings. The only problem now is Creative's EAX was made obsolete in 2007 with Windows Vista because Vista axed the one thing EAX relied on: DirectSound.
That and people use OpenAL now or have software mixing that works with what Windows is set to. Source for instance has built in surround sound. Unreal Engine 3 games seem to know how many speakers you have as I've been playing Mass Effect 2 with surround sound without explicitly telling it to run in surround sound.
You only buy a sound card now to either get the best audio quality ever, or to get high-end audio features. And I could argue that to get the best audio quality, you would use S/PDIF with a TOS-LINK cable to a compatible receiver/amp. The audio in your computer is digital until it hits a DAC. Why not make it immune to noise (it's digital, small amounts of noise will not affect it, not to mention TOS-LINK is optical) until the very end?