They're still worth the investment under certain circumstances.
The first one is the obvious one. The onboard sound fails or there is a problem with it. In my case I had to buy a cheap card since for some reason, the onboard sound wasn't compatible with Windows. It would play sound, but the line-input and the mic-inputs wouldn't work at all. At least under Windows. Under Ubuntu it worked fine, but the drivers from the manufacturer's website were rejected by Windows and the built-in ones only ran the sound output to the speakers.
Since I needed it to all work (was using a chat program when I was playing WoW), I had to buy an inexpensive sound card and disable the onboard to fix the problem.
The second one is also obvious. If you're doing something with audio on a professional level. Things like mixing or sound processing.
The third is a little less obvious and one of the reason that I want a high-end card. Since I don't buy Cable Television, I have a media-PC running things. It's the hub for Netflix, Hulu, Amazon, as well as my Blu-Ray/DVD player. Since it's the hub of my multimedia system, I want it to run through some decent speakers and get reasonable sound. Since I'm in the IT industry, I can get a set of 7.1 speakers and a Creative Labs card for less than I could get a decent surround system at Walmart, Target or Best Buy.