That would be the case with USB 1, but honestly, I really doubt that a Carbon Copy Cloner backup will saturate USB 2. It's spending so much time reading small chunks from the internal drive and then sending them down the pipe to the external drive that the pipe is never going to get totally full.
It most certainly will. When it encounters a large file to back up and determines it doesn't exist or will be replaced entirely, it will push the drives to their limit on speed. Expect 18-39mb/sec depending on your USB bridge chip. 18, 26, and 36 MB/sec are the speeds of the most common sata to usb bridge chips, I've only seen 39 on OWC brides and I'm not sure how they're pulling it off.
I watch large file backups quickly flat top activitymonitor here at 79mb/sec on firewire800.
The big thing though with backups is that you don't do that very often usually, and backups typically run during off-hours like automatically at 2am, where speed isn't too important as long as it gets done before morning. So a slow drive, or even on a slow connection or even over the internet isn't a big deal.
I don't personally feel a faster hard drive matters much, but I know a few people that insist on faster drives in their mbp's for boot camp and vmware/parallels use, apparently it helps quite a bit there going from 5400 to 7200. Though even I wouldn't turn down a cheap large SSD
