In my mind, the threshold for sound adequacy for the average person has been reached with onboard audio.
The expanding market in terms of HD and improved display graphics continues to expand. I agree forking out top dollar for the newest card on the market may seem futile, but some like the best, and in doing so they provide, in some sense, a form of future proofing against games that come out with better resolution and more demanding graphics before they upgrade their entire system.
The onboard audio vs soundcard and onboard video vs video card arguments are not apples to apples in this case. I think most would agree they would play a game with average sound and great video before they would play a game with average video and great sound.
The marked future proofing improvement of spending $100+ dollars on a GPU vs a soundcard make this a no brainer for any gamer as audio quality with onboard audio is more than adequate for most users.
The bottom line is that the marginal improvements in graphics beat out marginal improvements in sound any day. Video is more important that audio. In choosing a gaming system that contains onboard audio + GPU or onboard video + soundcard, I, and many others, would taking the GPU and onboard audio hands down. For gaming the cost of upgrading to a soundcard is just a misuse of money as you could have spent it elsewhere in your system to get better performance instead of the marginal upgrade in sound quality.
In the build forums here, it is rare to see someone suggesting $70 dedicated to getting a soundcard to anyone on any kind of budget gaming PC.