Power users? No, if by that you mean the traditional computer geek.
Gamers - yes, unfortunately the directional sound (and EAX) support of onboard audio is still not up to the same level as Creative's. This is especially relevant if you play FPS games with either a 4+ channel speaker set or headphones. The X-Fi's headphone processing combined with a quality pair of headphones wipes the table with any of the so-called "surround" cans.
Hi-Fi enthusiasts - most definitely. Onboard audio is getting better, but not quite there yet. Most often the rare onboard audio codecs that are of high quality are only found on very expensive mainboards. It's often much more practical to get a cheaper mobo and a proper separate audio card.
Musicians, DJs and others - most definitely. Need multiple independent channels, ASIO drivers, proper A/D converters, etc.
Other potential applications include HTPCs. Take a look at Asus' new Xonar HDAV 1.3, for example.
However, you're right in that the market has shrunk considerably. Onboard audio is good enough for most people these days, and Creative's sound card business is in trouble. For me, an M-Audio Delta with a Creative X-Fi (one of the real ones, not the XtremeAudio fakes) is good enough. But I'm picky compared to most when it comes to audio.