Here's an alarming statistic: 50 percent of all iPhones taken to the Genius Bar have never been synced. Of course, that's a certain percentage of iPhones. Those taking it to the Genius Bar are obviously having some sort of trouble with their iPhone.
The problem here is that we live in an age where smartphones hold just about every dang thing we need: photos, contacts, music and so on. Not backing it up means that losing your iPhone or somehow having it wiped would mean losing all of your stuff, and you'd have no way to recover it. Nothing would be backed onto your computer.
According to a former Macworld editor, a source let him in on that bit of information and says that this is one of the main reasons the folks working the Genius Bar are excited about iCloud.
iCloud of course will automate the sync over wifi and back their data onto the cloud. Those who don't back up will be very happy for this feature if their iPhone dies, even though they probably don't even know what the cloud is.