The phone is syncable by USB, it's just that there are bugs in Apple's USB drivers (particularly in 10.4.x) that prevent iSync from being able to detect many phones via USB.
Personally, I find Bluetooth syncing far preferable to cable. No tangled cables to find and plug in - just turn on BT and click Sync. You can even set it up to automatically sync when your phone comes in range of your Mac, then you don't even need to press a button.
Then there's all the other cool things you can do via Bluetooth (SMS texting, making and taking calls, hands-free calling through the Mac, remote control of your Mac etc. etc.) none of those can be done via USB.
And you don't have to worry about losing the cable!