How often do the two servers replicate? I have a client that has the same type of set up, Active Directory on a server that replicates to another server every 5 minutes over a fiber channel.
I still have them include the Active Directory into their nightly backups just in case. You never know if something bad will replicate to the other server, then you just have two bad instances of the AD.
We would recommend at least having it backup weekly at a minimum, but daily would be preferred.