Its mainly to lock generated structures in place like witch huts and nether fortresses, otherwise they would generate according to the worlds seed and not where they've already spawned pre update
We will hopefully update tonight. I wish mojang wouldnt of did the release this way... There was no reason to block 1.6.3 clients from connecting to 1.6.2.
mine says 1.6.4! and also cannot connect to the 1.6.2 client. even when reverting to a user profile of 1.6.2 really odd