We need soundcards because mobo-based S/PDIF in/out was uncommon, historically. It's becoming more common, though I wouldn't go so far as to say widespread. All else being equal, PC w/ mobo-based S/PDIF output > DAC would work just as well as PC / soundcard with S/PDIF out > DAC. But I'm guessing all else is not equal - if it matters to you, find out if the mobo-based S/PDIF in/out resamples on the board you're considering. I suspect it does, while most soundcards tapers elect use do not.