I had a dream once that I saw Justin in a grocery store and started to ask him about this bug before he cut me off. :-D
If this issue were corrected, it would make drum programming workflow easier from a visual clarity standpoint, IMO. Right now, if you only want the editor to show you a few rows, the rows become huge and unwieldy. If the rows were able to retain the same size as before the extra rows were removed, this would really help usability. Like if you wanted to have the midi editor docked and only need to have a few pieces named for kick,snare, hi hats, ect.
I think this is the last piece of the puzzle for a truly great drum editor mode in Reaper. It's like 90% there, but this bug is a pretty big deal, I think.