Duplicity provides incremental backups; it doesn't provide snapshotted backups. Snapshotted backups are the "best of both worlds" between full backups and incremental backups -- you get the semantics of full backups (each individual backup can be restored or deleted without touching the other backups) along with the efficiency and performance of incremental backups (if only a few files have changed since your last backup, taking a new backup is very fast).

It may be that duplicity is adequate for your needs, but I did consider it before I started writing tarsnap, and decided that duplicity wasn't good enough for me.