An USB dac isnt necessarily better for computer use, IF you want to use your system for gaming also. If you have good soundcard for gaming, like X-Fi, you can plug normal external Dac to its digital output and there it is. It works in all modes, no need to switch cables from USB dac to soundcard and vice versa. On gaming mode the soundcard does all the necessary accelerations, 3D calculations and so on, and this signal then skips the built-in dac and opamp to external DAC, which turns it to heardable analog sound in better quality than mere soundcard. High-end gaming audio!
On X-Fi, and X-Fi prelude, its very plug-and-play installation, so easyness of installation isnt really a reason to go for USB dac on pure desktop system if better device is available.