RP has some issues, and Cakewalk is more than aware of them. If you're going from Rapture to Rapture Pro there's much less difference than going from Dimension Pro to Rapture Pro.
Rapture and Dim Pro were getting long in the tooth. I've had stability issues with Dimension Pro and I expect that at this point, they won't get fixed because they're too random. It looks to me like there were five main goals with RP:
1. Not have the artificial division between the two instruments.
2. Update to a more modern code base.
3. Not obsolete existing expansion packs so people could do everything with RP they did with Rapture and DP.
4. Make it easier to use in live performance, or for "live in the studio" fans
5. Provide a modern platform for future content.
Those all make sense, and I suspect when Cakewalk gets the tech issues sorted out, RP will be a "workhorse" kind of synth. I will say that playing with it has reminded me just how much good content there is between Rapture and Dim Pro.
The biggest roadblock may turn out to be finding out why performance varies so much according to different people. Some say it loads really fast, some say it's painfully slow. BobF's comment about how he sped up load times dramatically by excluding the content from his virus scanner goes to show that we'll probably have to figure out a new repertoire of system dos and don'ts for RP, just as we have to for SONAR and other Windows programs.