I generally tie story's to more hardcore games yeah, you can't just pick up a book and start in the middle after all lol.
Casual games are great because they're entire experience comes from the game play, you don't need the flashy shebang and what not. I can easily spend 5 hours on rock band and not once even think about a story or purpose :P The same can't be said about Bioshock :x
In your case I'd say a game like Monkeyball can be considered hardcore up until you get everything, in which case the game becomes what it was meant to be. A casual experience. You can down an infinite amount of hours on a casual game and it never changes. That's it's point. You play the mechanics not the plot.
From 0 to KICKASS in .stupid seconds.







