Just because the number of changes is relatively small doesn't mean that each one doesn't drastically change gameplay.
hmm... I think people loose perspective here somewhat. Just on the surface:
Entrenchment $10 - Adds Star Bases (x3), Mines, Vasari Mine Layer, Turret&Hangar Upgrades
Diplomacy $10 - Adds Envoys and Pacts (x3)
Rebellion $30 - Adds Titans (x6), Capital Ships (x3), Corvettes (x3), Vasari Nanite Swarms, New Research (which unlocks new strategies), New "actual" Victory Conditions, Shadows.
Thats 36 abilities on the new Titans and Capital Ships compared to 12 with Entrenchment. These of course include new particle effects and sounds.
It's well over 40+ abilities when you count Corvettes and station improvements.
The new Research is far and above anything Entrenchment or Diplomacy added. A surface look won't reveal the complex nuances added by the designers.
Then you get to the subtle differences...
While the optimizations are expected with each release, the graphics improvements and shadows is a new feature and really adds to the game.
Updated particle effects for all 3 races from phase jumping to weapons, etc!
New UI HUDS for Advent and Vasari increasing your immersion into the gameplay!
This expansion unlike the previous two added far more unit sounds and effects.
New significant modding potential.
Probably some other stuff, but no point in going on.
In short Rebellion is well worth $30 content wise compared to the previous expansions you seem to feel where worth $10 each.