So just to be clear, Rebellion will include all previous Sins content, I just won't be able to play the older stuff until release of Rebellion?
All expansions of sins including rebellion comes with the previous content. The difference with Rebellion is you do not get separate executables for Vanilla, Entrenchment and Diplomacy. Trinity however includes 3 different executables for the original game and each expansion.
One way to look at purchasing Trinity right NOW is it will only cost you $10 IF you plan to buy Rebellion as you'll get $10 dollars off that purchase. The advantage here is you will get all 3 executables as well as Rebellion (assuming you pre-order now).
Having all 4 executables opens you up to a wide range of gaming possibilities which GoaFan pointed out.
Diplomacy is currently the MP standard (might change with rebellion).
Mods that MAY make having all 3 executables worth your while:
Dawn of Victory (original version, not released but looks good)
Sins of the Prophets (Entrenchment)
Sins of a Galactic Empire (Entrenchment)
7ds beta (Entrenchment)
Maelstrom (All versions)
Distant Stars (Diplomacy)
Star Wars Requiem (Diplomacy)
Sins of the Fallen (Diplomacy)
Sins of the Fallen: Blood and Chrome (Battlestar Galactica) (Diplomacy, work in progress)
Stargate Races (Diplomacy)
Stargate Invasion (Diplomacy)
Star Trek: Sacrifice of Angels 2 (Diplomacy)
Babylon 5: Sins of the Younger Races (Diplomacy)
Goa's 4x mod (Diplomacy)
If you like mods the released mods alone are worth the extra 10 bucks.