You’re unlikely to have much data on your iPhone that isn’t synchronized from a computer; so do you really want to spend minutes waiting for iTunes to back up your phone every time you plug it in? I know I’d much rather lose some stupid game saves than wait ten minutes before I can resume listening on the computer to the podcast that was playing on my phone when I get home.

This might not be a big deal if you only plug your iPhone in to charge the battery and don’t need to sync often (in which case you probably use the AC adapter anyway), but here’s the simple Terminal instruction to disable backups:

defaults write com.apple.iTunes DeviceBackupsDisabled -bool true