I have an older Creative X-Fi card in my system, and it's really only there for one reason: so I can record audio from, say a film clip into an audio file. You can only do that if you have two sound cards, or in my case built-in audio and a sound card, since it uses one for output and the other to record that output. I don't do it much, but it's nice once in awhile to have the capability. That being said, the main reason I'm not especially interested in a separate sound card is simply that I'm not willing to spend the money on amazing speakers.
The speakers I have sound the same on the built-in vs. the Creative (they're 400W logitech THX something-or-other, the ones with the giant subwoofer & 4 satellites). That being said, I *do* have them plugged into the Creative card. I wouldn't turn down awesome speakers if they fell in my lap, but what I have sounds good enough and for me it's not worth the extra money to upgrade. I'm also not overly picky about audio quality - now that hard drive space isn't an issue anymore, my mp3's overall are larger files than they used to be, and I don't notice the quality loss compared to uncompressed.