Finally finished the main game's single player campaign the other night. I will get back into multiplayer and grind to 50 soon, but I'm kind of waiting for my one friend to get the game so we can co-op.
I didn't like the ending nearly as much as the ending of ACII. They were trying to go for a "wtf" moment like we had in ACII, but went a little too far down the conspiracy/complexity rabbit hole for their own good. Imo. I'm guessing half the questions raised will never be fully answered by the series (since there's so many), which doesn't bug me in principle, but in execution usually does. What some games/movies/books play off as depth is really just laziness and/or an excuse to seem deep by leaving too much sh*t open-ended (see also: Lost, for a good example).
Anyway, bring on ACIII. DLC is filler. I'd just as soon have them stop releasing content and get to work on the next full title.
Eventually the modern-day story will be the only story (or one would assume). I'm not sure ignoring it is the best policy. And with the amount of time spent in the Animus, it's little wonder the historical material is more fleshed out. That will undoubtedly change.
Of course I'll play the DLC. I just don't necessarily need it. I'll enjoy it, it'll be fun, but it will still probably end up feeling like filler.
I ignore it because I find it uninteresting and the characters just very...flat and one-dimensional. The historical part of the plots is far more important; Ubisoft has even said that they plan on keeping the historical side of things the main focus. So yeah. Don't think that ignoring Desmond's 'plot' is going to really be a big deal.