I like the MB3. I'd like to see it in the recording time line and in the piano roll.
edit------
I didn't do a great job explaining this feature the first time, but the more I use MC5, the more I see some cool usages for it- so take two:
MB3 means a user can click on the scroll wheel of his mouse, then scroll the pointer horizontally or vertically to another position within the recording or "sound" time line.
I have found it quite useful in pdf documents, music applications and web sites with a lot of information that I need to navigate back and forth to....such as forums. Try it right now, it's awesome! I use it almost to the point that I am way too lazy to grab a navigation bar anymore.
Hope that explains it better!