I get the feeling the main purpose of that dlc was to give mass effect 3 a reasonable beginning. we aren't supposed to make any decisions here.
in "arrival" we learn, that after the destruction of the mass relay the reapers might need a while to find a new way out of darkspace.
my guess is that during this time, shepard gets convicted for the destruction of a whole system in order to prevent war with the batarians and is locked away somewhere.
when the reaper invasion finally begins, shepard is needed again. so, as always, you will have to get together your new/old crew, recapture your ship (which will probably be the normandy sr-2, as EDI has already been confirmed for mass effect 3) and kick the reapers $§%§$.
at least that's what it looks like to me. I just don't believe they'd mention the probability of a military trial against shepard if that trial wasn't supposed to happen, besides we know mass effect and bioware games in general.
getting together a crew and doing some assignments for them was an important part of mass effect 1.
in mass effect 2 getting together your team and gaining their loyalty even made 70% of the whole game, they won't abandon this in mass effect 3.
the only thing they needed was a reason why shepard, AGAIN, would have to get together his old crew or a new team and this reason might be given with "arrival".
i mean i don't really like the idea, i like my ship and my crew and i hope most of them will return back in mass effect 3 but really:
which bioware game did ever begin with the hero having a powerful spaceship under his command and a crew of loyal companions who would follow him/her into the deepest pits of hell behind him/her?
quick overview [MASSIVE SPOILERS]kotor 1: collect some followers, steal a ship, collect more followers, help followers, get a little romantic, save the galaxy
mass effect 1: collect crewmembers, become a spectre and get a ship, collect more crewmembers and help them with one or the other problem, make out with the blue girl or the redneck (if your shepard is male), save the galaxy (for now)
dragon age: origins: become a grey warden, travel around and seek help against the archdemon, meet companions who are willing to follow you, help them with some problems, perhaps make out with one or two of them, slay the dragon and save ferelden
mass effect 2: die, be resurrected 2 years later, get a new ship and a new crew, gain their loyalty by helping them with their problems, make out with one of them or stay with the blue girl or the redneck (again, if you are male), save humanity (for now)
dragon age 2: get to kirkwall, start a new life and get some new friends, help them with their problems, beat the crap out of the arishok and become champion of krikwall, help your new friends a little more and eventually make out with one of them (really cosmopolitan medieval society we have here), beat the crap out of the crazy templar and save....well...nothing this time....
mass effect 3: hmmm...get out of alliance custody and get your ship and crew back? help them with some of their problems to gain or regain their loyalty? make out with your romance from me1 or me2 or even with a new one? finally beat the crap out of the reapers (or help them destroy the universe...please, i want that possibility

)?
DON'T mistake this for harsh criticism, all those points i listed are reasons why i like bioware games, as creating cool and interesting characters and developing relationships between those characters is definitely one of biowares biggest strengths.
ok, might have gone a little bit off topic here but long story short: although i liked the dlc and was quite thrilled, i guess the main purpose was to create a convenient initial situation for me3 which will allow bioware to use their typical formula of story development, there aren't supposed to be any decisions.
and it might also serve as a convenient beginning for those who haven't played me1 and 2: former alliance hero gone rogue is called for help to save the galaxy from a threat only he knew for sure existed, something like that. what more does a casual gamer who doesn't know the whole story need?
but perhaps i interpret too much into these last scenes of "arrival", who knows