And I'm sorry, Verno, but they didn't go through all this much trouble with the Rachni to just not take the story anywhere beyond an email. I just don't buy it.
Err where exactly have they taken it? In the second game I encountered a single NPC who gave me about 5 lines of dialogue related to them. Did I miss some big Rachni event or something? Looking at the "consequences" from the transition between games the highest impact I've seen is a missing character so far. I'm sure I haven't seen all of the possible choices so perhaps other people have seen significant changes? If so please by all means, lets hear them.
I think the big thing you're missing is that this is the final chapter of the Shepard trilogy. There are still limits as to what they can do, in order to keep people from being completely boned based on their prior decisions, but they have a lot more freedom here. They don't have to worry about importing decisions to a fourth game.
The "emails" and what not were simply ways to acknowledge choices that were either very minor and had no lasting impact on the story, or that would not pay off until ME3. The alternative was to not acknowledge them at all.
It stands to reason that not every choice you make has galactic implications. But this being the final game gives them a little more freedom in what they can do.