If it sees a longer chain, but it can't process it, then it knows something is wrong. It displays "WARNING: Displayed transactions may not be correct! You may need to upgrade." and makes most RPC commands return an error.
What sort of attack is this intended to protect against? If it sees a longer chain but can't process it, it should also fail to validate any of the transactions in that block, right?
I presume mainly just to bring up an alert that an incompatible change is running on some nodes, weather it is a new version from Satoshi or someone else is trying to change something either way it'll be nice to know.