Finally beat the game, though thanks to reloading a save I've seen all 3 endings.
On the matter of "your choices didn't mean jack squat!":
Of course they did. All the choices you made throughout all 3 games were in some way or another leading to the formation of the grand galactic fleet that engages the Reapers in the final battle. Their entire purpose was purely to get the Crucible to the Catalyst and make the damn thing go boom. Your choices are what determine what allies make up the composition of that fleet. No one knows what'll happen when the Crucible is fired off, they just all have faith that it'll somehow get rid of the Reapers. In the end, all the choices you made were to build a fleet strong enough to ensure that Shepard is standing there at the end with Spontaneous Combustion Boy.
Can't say I really liked the whole "Ending-o-Matic" thing with being able to pick which ending you wanted, but when you think about it: ME 2 ended the same way. Destroy the Collector base or keep it.
Personally I found all the endings to be acceptible...
I knew that Shepard was most likely going to end up going all Neo on the galaxy and making the Jesus Christ sacrifice to bring about peace. If you didn't see the sacrificial Shepard coming, then you're quite blind.
I do agree, though, that just a bit more closure would have been nice, something like what they did at the end of Dragon Age and have text explinations of how the choices you made ultimately effected everyone, the way DA tells what happens with the dwarves, elves, mages, etc.
Would I have liked to see a more fulfilling ending? Sure. But I'm in the apparent minority that really wasn't outright offended and insulted by the ending. Personally I got all the flavor of an ending that I needed when I was nearly brought to tears during the segment where you're going to meet with Anderson for the last time and everyone has the chance to say goodbye to you, yet another MASSIVE hint at what I mentioned in the above spoiler box, if you hadn't caught on already.
Seriously, everyone's getting all weepy and saying their goodbye's like they know you're about to die...even when you assure them that you're going to go kick some ass all the way to the finish line, they still insist on saying goodbye like they'll never see you again. It's called "foreshadowing" folks.
As for the topic at hand...would I buy some DLC if they decided to change up the endings a bit? Certainly. I'm just one of the few that don't think it would be necessary.