This is a very relevant point. I have already surmised that ROM is a topic which cries out for clarification. When we say ROM, what do we mean ? Do we mean move the weight as far as we can ? Do we mean move the joint as far as it wants to go ? Do we mean (and here's what you're hinting at) as far as the muscle NEEDS to be moved to induce strength, hypertrophy, etc. ?
Back to pullups, I do agree that if hypertrophy is the chief concern, I see no need to get the chin to the bar. As far as I'm concerned, that's just a contrivance.
Of further interest, Aaberg recommends, as you do, pulling only until upper arms are parallel to floor, maybe a little more. But muscle tension is paramount.
It does so happen that I am able to maintain lat tension, all the way up, for at least a few reps. But again the conventional advice would have me either terminating the set at that point or kipping a bit to get another rep or two. No consideration is given for a calculated decrease in ROM.