I have Rapture, Dimension Pro, Rapture Session and Zeta+2. Any reason to get Rapture pro besides the
GUI?

i don't use it much, but that is mainly because of the bad taste left from the state it was released in, and remained in for many many months after. it now works fine, has many good sounds/patches of it's own, plus you get rapture and dimension pro stuff all collected together under the 1 roof. if you have a lot of other synths like i and many others have it may or may not be that appealing to you. try it out, see what you think for yourself, after all your opinion is really the only one that matters in the end.

I have a lot of fun with it.

Rapture Pro does have some features that make the upgrade worth it. 

The ability to search for patches
The ability to have favourites
It will reuse waves that have been loaded from previous patches (if required), rather than reloading them
Having all your patches from Rapture, Dimension Pro, Rapture Session in one synth

Note: I would not deem Rapture Pro a replacement for Zeta+2 - for instance, Zeta+2 supports polyphonic aftertouch events (each note of a chord can an independent intensity of effect) whereas Rapture Pro treats polyphonic aftertouch events as channel aftertouch events (The effect operates on all notes of a given chord).

These are a few things

What I would suggest id try the demo.  It's really an individual thing.