I've found that onboard sound is much more susceptible to noise generated by other components/connections to the motherboard. For instance, on my work computer, if I plug headphones into the onboard sound headphone jack, there is always a background buzzing noise, which increases when I move the mouse.
Similar things have occured with friends who use onboard sound...many times there seems to be more background noise. Some find it too much to take and purchase a sound card, other's ears are not as sensitive and they don't worry about it too much.
I happen to have very sensitive ears to that sort of thing, and thus I use a sound card. Yeah, it is a bit more money, but there are other benifits to it.
When I build machines for friends, who usually are trying to get the best bang for their buck, the usual choice/recommendation is to forgo a dedicated soundcard in the beginning. Then, if increased sound performance is desired at a later date, or if background noise is too much for them, they can always drop a soundcard in whenever they want.