I've had issues using s-video in the dim distant past because IIRC I had to actually specify the input signal of the card somewhere in the device driver. The fact that audio had to be handled separately also made things more difficult.
That being said, at the time it was worth the effort because I got visibly better results from my VHS recorder and I was paying a premium for S-VHS tapes. The few extra lines of resolution made a difference.