Because why not? Most games have an offline/single player mode save for MMOs. Games that aren't MMOs that have eschewed a way to play offline tend not to do too well.
At least with Crackdown it'll be really easy to tell exactly what benefit the Cloud is giving it. They'll undoubtedly make that playable offline.
Most games do yea, but most games also don't have any notable cloud implementations. If there is an offline mode for something like Crackdown, I'd expect the differences to show in whatever was being offloaded (e.g. physics interactions) rather than the graphics. I wouldn't expect them to be like "right, the cloud's gone, do the same calculations locally, and drop the graphics". I just don't see that ever being a scenario that happens. Whatever is being offloaded would most likely simply not work locally at all.