Most backups are never actually read / used after writing but /dev/null is not a good substitute for them.
[Edit] in case I wasn't clear: Imagine data that is written and rewritten and overwritten before it gets read; a user that never returns to your product - his state data was written but will never be read. That sort of thing.
I assume that you mean write once data. If you mean write only you might as well use /dev/null.