Well, it DOES matter... most consumer cards like the earlier soundblasters had a rather high noisefloor. Mixing down multiple tracks thru the same noisy inputs would drastically increase the noise levels especially if you add compression to the mix.
Really good cards like the Indigo and the Audiophile have very low noise floors allowing for a much higher head room when mixing. Even if you don't use keyboards and other instruments, you may want to track vocals at one time and that's where you will realize the importance of a very low noise floor.
Also, the quality of the cards converters determine how good a recording will sound. Having very good to excelent converters in the A/D section is the holy grail of digital recording because poor converters often sound brittle. Conversely, when mixing down, the cards D/A converters come into play because you will then need to hear each and every instrument in a mix and be able to place it in an imaginary 3d space, so to speak. Poor D/A converters will often sound muddy, brittle or lack clarity and depth.
hth,