I'm gonna have to disagree with you here, Evan. First, if you use your pad as a virtual frame it would be a pain in the ass to have to take it out to sit on at rest breaks and then have to fit it back in your pack. Second, even if you don't use your pad a virtual frame, it is much easier to pull out a small, conveniently packed sit pad than to dig out your sleeping pad. Also, your normal sleeping pad can be supplemented w/ your sit pad to increase the warmth and comfort of your sleep system.
EDIT: sry to repeat many of Dave T's points...i posted this before i saw his post