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Replication doesn't defend you against deleting the wrong file, or messing up an update to your database, or any of a large class of PEBKAC issues.

It's also not useful if your main files get corrupted and you diligently propagate the corruption. See: ma.gnolia

Setting aside the database, file problems are fairly easily solved via svn, .snapshot or a good configuration engine.

As far as the database goes, I'm big on stored procs + archive tables, but i'll leave that to the grown ups ;)

...until you forget to check in important changes, or neglect to add a seemingly trivial configuration file to your SCM.

Whereas, a policy of automatically backing up everything except for your exclude list would have saved your bacon.