Having recorded both live performances, and manually tracked things in Reaper's midi editor, for me, there really has only been two things that I, personally have missed / wished it had.
1) Delete duplicate notes.
I was experiencing an oddity where (I believe after quantizing) duplicate notes (same start position, usually same end position)
2) Delete tiny notes.
Recording a midi kick I was noticing a bucketload of these midi notes. In hindsight I probably had my TD3 set to wrong trigger type but, I really wished I could have easily done away with them.
That said, I dunno, I'm a musician, and a programmer (uh oh!) And I find myself being rather proficient in the midi editor. I come from the days of using FastTracker
and I have to say it's miles ahead of tracking
I can whip up drum beats very quickly, cutting and pasting away, creating glued chunks that I then pick and choose to make my song.
$0.02 thrown in, keep the change