Rebellion is a stand alone release but it is basically built on top of the other games, there is pretty much nothing in the other games that is not in trinity. So no, there really is no point in getting the older games. 
That said, if you really care about the story, you may want to watch the intro videos for the Original Sins of a Solar Empire and the Trinity edition on YouTube, they aren't as awesome as the Rebellion intro cinematic but they might have more details about the earlier parts of the game's story.
