well, when you launch the diplomacy exe, you have diplomacy incorporating the features of sins and ent...
It is maybe a good idea from Stardock that Rebellion will only allow to play Rebellion... community is already enough fragmented as it is... and each .exe have some improvement who was never ported to the previous version... by example, some of the TSOP optimization was included in Diplomacy but not in sins or ent...
Rebellion will be the last release for sins, good idea to consolidate everything in a single .exe..