The main reasons for me to choose duplicity is the multiple protocol support. I've had so much issues in the past switching to other storage facilities only to have to completely change the backup process. Plus I'm using multiple storage sites for really important data.
Duplicity can be a bit finicky though, especially when it comes to cleaning up after itself, even more so if the backup was somehow interrupted.
Duplicity is also unable to incrementally remove old backups.
A backup set starts with a full backup, then has incrementals after that. You can remove the incrementals, but if you remove the (old) full backup, you have to do a full backup again.
A typically strategy is incrementals every day, and a full once a week. However this is really hard on home connections - it's just too much data to do a full backup every week.
One of its pros is that everything is completely encrypted from when it leaves your machine - so you can backup to a completely untrusted repository without worry.
However, in order to do that it locally stores block-checksums of every file it backs up. (So it can detect differences.) These files can get large.
Another option from the same source is rdiff-backup. This has none of those limitations - it uses a CVS style reverse diff (so old incrementals can simply be deleted, and the most current is simply stored as a file, which makes restores very easy). However it's not encrypted from the source - so you have to trust the repository at least enough to create a local encrypted volume on it.
Duplicity can be a bit finicky though, especially when it comes to cleaning up after itself, even more so if the backup was somehow interrupted.