The more I think about it, the more it makes sense... why treat pullups any different than any other weight training exercise? I can remember getting pinned under a 115-lb barbell during one of my first benchpress workouts... a couple years later, maxing out around 200, I could do at least a couple dozen reps with that weight.
Pushups are different, because they are so easy that 30 or 40 reps takes very little out of me, so I can train those everyday. But I wouldn't train a heavy benchpress or deadlift everyday. Why do it with pullups?
Thing is, significantly increasing my max pull will take a while. And I'm cool with that, but what about all those folks that claim they went from 4 pullups to 20 in two months just by training them everyday?