I had never really done this, but I had to stop for a moment and say, “Wait, I did nothing of the sort! There are hundreds of other hero’s here that are doing the same thing!” And when I rode off on my speeder, dodging blaster fire from all the exact same mobs that I “saved them from”, the feeling was even emptier.
What you're describing is mimesis-breaking and it's common in games but in MMOs -- particularly those that strive for story -- it might be impossible to avoid. After all, on Korriban I can see twenty Khem Val's running around. Nothing kills immersion in a story than something like that -- if you go solely for that kind of immersion.
When you go to a planet, no matter how many mobs you kill, they always respawn thus it seems like you have no effect whatsoever. Whatever you just blew up: it's still there for the next player. The big bad boss you just killed: he's back again. The same guys I just helped from a crashed spaceship are STILL at that crashed spaceship.
A consequence-persistent world that never breaks immersion seems unlikely in an MMO because it's a shared space. The events of consequence only have relevance (from an immersion standpoint) if you don't notice that everything you just did has been "reset" save for your memory of the event. I suppose the good thing is that the area where you did your story deeds is often closed off once you've done them so at least that part is not thrown in your face.
I have to admit that as a new player to MMOs this is something I initially struggled with since nothing seemed like it really mattered, given that everything was pretty much in the same state after as it was before.