The industry has obviously jumped on DLC, probably as a way to increase revenue without increasing the up front price of games, and Nintendo has gotten into the game especially with NSMB2, Fire Emblem and the upcoming NSMBU Luigi. I think, because of Nintendo's specific situation, DLC can be a great thing for them.
Nintendo's key franchises are huge sellers, but it's difficult to stray far from customer expectation. This can result in loss of novelty. DLC is a chance to do something new, introduce new villains, and gauge customer response. For example, people may have been a little disappointed (and not purchased it) if Bowser and his children hadn't been the main bad guys in NSMBU; and if the general thrust of the game hadn't been along the familiar lines of SMB3 and SMW. The DLC is a perfect chance to try some really different things and perhaps develop fresh new villains. Customers already got the experience they were expecting, and the DLC isn't the price of a full game, so they're more inclined to try and enjoy something new.
Similarly, DLC is a great chance to add value to an existing game. If they have more level ideas than would fit in a Mario game, or a dungeon for Zelda that just doesn't fit with the others, DLC is a way to still get it to the customers. It's better than putting out a whole new game that is too much like a rehash of an existing one, and it can be offered at a lower price but with higher profits because they don't need to make physical copies.
The ugly side of DLC is if companies put out games poor on content or even incomplete, and then sell DLC to extract more money, but I don't expect Nintendo will do that. They characteristically include a lot of value in their first party games.
— working underground to save the castle from Bowser plans before they even happen...