This scrollbar should support Pinch Zoom for the pattern editor (ModStep currently does this, and it's a great feature for workflow).
You can easily pinch zoom both horizontally and vertically right in the pattern area. Why try to pinch in a tight scroll bar when you already have the larger area under it?
Couple of reasons.
1) A lot of the time I just want to make the timeline shorter/longer - but I'm happy with the height (e.g. I'm editing a drum pattern).
2) This is how the note/pad labels on the left of the pattern work, so it would be more consistent.
3) It means I can zoom in and out easily without worrying about accidentally creating notes on the pattern.
And finally, it's just a way quicker and more efficient workflow. It's one of the things ModStep got right.