Great thread here! I guess I have to chip in, for whatever that's worth.
The very fact that I cannot select the notes that are beyond the staves is enough to drive me crazy. I don't necessary use notation to sequence because of realism issues. But it's crucial for:
1. Editing of individual notes. (yes, that can be done in PRV, but not my preference)
2. Global view of my arrangement and composition. I need to see everything to get my harmony, counterpoint etc right. It's not just listening, but also seeing it. You can't visualize it in PRV. As it is, it's a pain to my workflow as I have to resort to paper for certain sections of my music just to see my arrangements.
The only time I've tried using PRV was more than 10 years ago (on Cakewalk Professional 7). It's too limited for me and I've never used it since. PRV is basically a useless feature for me. I don't need notation at the level of Sibelius - I just need it to be
improved. That's not too much a request, I feel.