You certainly can pay to use Google Documents and do your own backups. It's just that doing so would defy the biggest appeal of the service. The appeal is that Google handles day-to-day operations for you.
There's more to "day-to-day operations" than backups.
The reality is that Google is much less likely to damage or lose your data than you are, but it's always good to keep extra copies around locally. This is important for many reasons, like access when either Google or your whole net connection is unavailable, and more flexibility with the data.
