After seeing the loop recording feature in edison, I thought something similar would work just as well for MIDI. Here's how it would work: The takes with preview checked are playing/looping in the background, and when you press accept, all previewed patterns are dumped to the piano roll.
Sounds pretty much like the riff machine I think.
If you screw up while overdubbing, you uncheck preview on the bad take and you can continue as if nothing ever happened.
You don't have to choose between overdubbing and overwriting, you get a flexible mix.
(overwriting: check "mute unlocked takes", uncheck "mute this take". overdub: uncheck both)
You can jam and record without making the piano roll a mess, and afterwards listen and choose among your takes.