While it is true Rebellion has all the previous content ever developed for Sins, unlike Trinity it is not backwards compatible. You can only play Rebellion, you do not have access to play trinity/diplomacy or the original game by itself.

That said, there are really only two cases where this is an issue. One is if your friend has one of the older versions and you'd like to play with them. The other is if there's a mod you want to try, though most of the mods are being updated for Rebellion. If neither applies to you I'd say you aren't missing out.

Rebellion.

No. That's what I've been trying to say. It is not backwards compatible. You can only play Rebellion, you can not play the previous versions without the Rebellion content. Trinity had that but Rebellion does not.

What exactly are you worried about that you'd want to play the old expansions?

If you buy Rebellion, you have every contents from the original game, Diplomacy and Entrenchement, plus new content (like subfactions, corvette, titans). So if you want everything, go ahead and buy just Rebellion. The only thing you can't do if you have Rebellion, is playing online with people who only have Trinity