I don't think the comic book crossover comparison really works, this is an add-on for a game you already own, not a separate game in a different series. "Look out for the exciting conclusion to Mass Effect: First Contact War in the upcoming Dead Space: The Delimbening!"
The problem with this is that the DLC doesn't contain the "real ending" to ME2, in fact it's more like "telling a short story in the gameworld". Both LotSB and Arrival are side stories that aren't connected to the main story of ME2. Arrival is more of a prologue to ME3 than an epilogue to ME2.
That's kinda how I see it. Considering Shepard is on trial for the events of the Arrival DLC, it's not like they are going to skip over important details and assume you already know what happened.
I think Lair of the Shadow Broker is actually a bigger issue for the I-don't-buy-DLC crowd as far as canon is concerned. "Okay gang, let's go pick up
who, by the way, is the
.
Anyway, Bio is apparently doing something similar to the PS3 ME2 motion comic for ME3 that tells you the story so far and lets you make decisions if you are not importing a save game.