Is one and done always meaningless?

Well, you do play to win the SB. I don't think it's as black and white as this though.

The Browns 07 season was not meaningless, nor was the 06 Saints. I would have to say that it's a case by case basis. I might even include the Giants in this too because after last year's melt down and chants of firing Coughlin have fallen to the way side and a "new" team has emerged as a dominant NFC team. Something that you can grow on with or without a SB appearance. I wouldn't concider their team's season meaningless.

Now take teams like Dallas and Indy. Their seasons aren't worth squat anymore after all the potential thrown away with their losses. That would be meaningless season IMO.