Its not even about quality or gaming. When you have IRQ conflicts because of onboard that cause serious hardware/software crashes, or you have interference from your Optical drives because they run on the same processor that your "onboard" is, or the fact that they literally fry themselves 75% of the time, I would think that buying a $20 soundcard would be worth it. When I can actually hear the whining and wind up of the HD platters, the network card, and all other parts in a PC, i consider it garbage.
"But Crysalis, my motherboard is 1337." Its AC'97. It is not good. No matter what you do, it is still AC'97 using the CPU's cycles as an audio processor. Do you use your car to heat your house? It produces heat... why not? It is just not meant to. A CPU is not meant to do audio processing. Compression, voltage, DSP, DTS, and all other forms of processing can not be done well.
And good point about 16 bit. Good golly... I could go buy a $20 card that has 24/96!! Onboard cant even do 16 without puking.
I dont really care if you are gaming with 12.4 surround sound, or if you have 2 speakers that play the little noises that paperclip in Office has; any audio processor is better than onboard. Period.
But... Not a one person here wants to hear it when an engineer such as ricanflow, EricB, or even myself says something. Its cool.