Well, i bouth the sound card because i needed to use the 5.1 speaker set i had, it has 4 inputs and not 3 like the motherboard (1 for center/subuffer, 1 for front speakers, 1 for back speakers and 1 for microphone)
I never made the moderboard to use all the speakers at all time, sometimes it used 2 speakers, sometimes the entire set (depending on what i was doing), but with a sound card things are alot easier and more eficient