Maybe the reason why Sonar's user base includes so few staff users is because music readers see no reason to buy it, precisely because it doesn't have decent notation functionality.
I for one, think it's a shame. The trouble with PRV's is that it's like the keyboard version of guitar tabulature. It's basically there for people who cannot read music, or people who can play by ear. I played guitar by ear and from tab for years. When I finally knuckled down and learned to sight read, it was a revelation. Now as a classical guitar player I cannot even imagine learning (or composing) an intricate piece with guitar tab. It doesn't offer a good enough visual representation of the music.
My main problem with PRV's is that I'm not a keyboard player. I can write music that's in my head, but the PRV is just nowhere near as good as traditional notation for setting down music swiftly and accurately, and keeping it organized in your head. If you have keyboard skills, then I imagine the brunt of your arranging is going to involve playing the parts on the keyboard and then touching them up in the PRV later. Without keyboard skills, you're reduced to inserting the notes on the PRV from scratch. It's just so much easier to do that on a proper stave.
I think that by offering a good quality notation view, or by offering a plugin for a price, Cakewalk would be greatly increasing the potential user base of Sonar.