You don't have to be physically connected to your computer to back up with iTunes. With iOS 5, Apple added Wifi sync to the iTunes syncing. It automatically happens whenever you charge your device and you can also make it sync anytime just by going into settings on your device.
Also, the iTunes sync is both faster and more comprehensive than doing it over iCloud. And probably the most important reason of all is if you use iTunes instead of iCloud for backup, you free up your free 5GB of space for email and other iCloud uses and don't have to worry about managing what apps are allowed to back up.
If you don't have a computer on the same wifi network that your iDevice regularly accesses is the only reason I would turn on the iCloud backup.
Edit: I guess if you only have a laptop, wifi sync is a little less convenient than my situation where my Mac Mini is always on and ready to sync but I think the benefits still outweigh syncing over iCloud.