What if you miss one of the timed event-only Pokemon? What if you want to try out one of the crazy rare monsters, like Speed Boost Blaziken? I would never take them online because of how unfair they are, but once I've finished the story Pokemon games get tedious quite quick.
And what is the deal with the dream world? I tried it, and that is a very strange way of letting people get different abilities on Pokemon. Having to go to a website on a computer and play a flash game to get something is equivalent to a Facebook minigame, and it's a boring, convoluted process.
I'm going to try and not get sucked into Pokemon again. I rebought a DS just for SS and B/W. Got my money's worth I guess, but I haven't touched the DS in months.