The only reasons to install DPro on a machine that will have RPro installed are
1) you plan on opening existng projects that use DPro
2) you want to use the virtual B3 that came with one of the DPro expansion packs. RPro can play the DPro virtual B3 programs but there is no way to use the features that depend on the DPro UI such as the drawbars and leslie speed. There may be other programs that use advanced DPro features like the virtual B3 but my knowledge of the DPro programs is limited. I have not used the synth very much.
3) you like the DPro UI.
Rapture Session uses a subset of RPro's programs and samples. I cannot think of a reason to install Rapture Session if you plan on installing RPro unless you have already created projects using Rapture Session and need to open them again.