Problem with ME3 DLC is that it won't have the weight of the previous ones. The main draw to buying ME2 DLC (to me, at least) is that it seemed significant to the story, so it was necessary for the sake of the conclusion to the series.
But now that draw's gone...you know how it ends, this is just extra gameplay. Which is nice and all, but in all likelihood, it'll just amount to getting +200 points or something of the sort, which is probably irrelevant since you can get all the endings without it.
That said, they do have a golden opportunity with Mass Effect 3. At the ending, they can have hugely varying effects from their DLC. Perhaps add new endings ontop of the existing ones. Hell, so long as the main three factors are consistent (Shepard Dies, Relays Destroyed, Reapers Gone) they have a lot of room to build new outcomes. Not as much as they claimed before release, but the chance is still there. It could turn into a big branching finale over time, if they stick to expanding on it alongside the pre-ending content.
At the very least, extending the end-game portions to reflect the outcome of the DLC would be nice, even if not particularly rewarding.
But that probably won't happen. It'll most likely be an addon that doesn't impact the finale in any meaningful way.
However, if Leviathan ends up taking out Harbinger before it hits, I will tip my hat to Bioware for an encouraging comeback.