As for the OP, if you like ME3, get the DLC: It's more ME3. If you don't like ME3, why would you ask?
As for the "this is so integral to the story that it should have been included with the base game," you kinda want it both ways: If it's not important, you say it's pointless and shouldn't exist. If it is important, you say it's necessary and shouldn't cost extra. That kinda defines away all add-ons that aren't completely separate narratives, and stand-alones are sold nowadays to stand... alone.