The Super Mario series rarely spends a lot of time on establishing detailed storylines.  Typically the games present a basic framing device to justify the action and then turn Mario or Luigi loose for some running, jumping, and shell-throwing.  The upcoming Luigi's Mansion: Dark Moon for the Nintendo 3DS seems to be headed in the other direction though, as the opening cinematic has made its way to the Internet a month before the game reaches stores and explains why Luigi is back among the ghosts.  King Boo is causing trouble again and when Professor Gadd finds himself in trouble with wild and crazy ghosts, he calls the only ghostbuster he knows via a fun Poltergeist reference.

Even this introductory bit is just a more elaborate framing device than we usually see in Mario games, but it's still nice to have it there.   I don't expect long, deep narratives when it comes to Mushroom Kingdom adventuring, but even just a little bit of entertaining explanation beyond "[Character] was kidnapped!" to kickstart events can be vital.