For me personally, it's an unnecessary update. It's just for switching to 1.7 snapshots & backwards. But i see no honest reason, why i should play my current map in a 1.7 snapshot and even if, why i want to go back. Every user can do a copy of the current map & test it. If it's bugged: Simply delete the copy and play again on 1.6.2. Sure: For some users - this will make sense, because they're maybe testing redstone-circuits & if they maybe will work at 1.7 too - just for easier switching. But else? I don't get the point.
I think, most of our beloved mod-developers will be really annoyed about. Some are updating & some will just think: "The hell? Why should i update my mod, if 1.7 is knocking at our doors!" and i really understand them. Many good mods are not even completely ported for 1.6.2, developers coding like they have no real life (for complex mods) & 1.6.4 shot them right into their heads.
I only recommend: Don't update to 1.6.4, since nothing really important has changed & 1.7 isn't that far from release-status. If you don't play with mods: Go ahead - you won't lose anything. All others - like i said - shouldn't consider updating. Especially not, if you want to keep playing your world with included mods. And please: Don't annoy mod-developers for updating their mods for 1.6.4: It's pointless. Thank you very much.
I found a new way to play 1.6.2 when you have updated to 1.6.4! Go to the new minecraft launcher, then click Edit Profile. Go down to Use Version: (and put in) release 1.6.2. Now you can play on older servers
Oh my God. That is the easiest thing ever in the Minecraft launcher, I think we all found that out ourselves
MC-15547 – Structures (witch huts, nether fortresses etc.) don't get saved with the world file
In order for current structures (Witch Huts, Nether Fortresses, etc.) to be saved to the world file, the player must load the appropriate chunks within the game manually. The game will not automatically scan worlds and update structure data globally. After this is completed, structures will be compatible in all snapshots starting at Snapshot 13w37b
Fixed handshake exploit
Verified by comparing the 1.6.2 and 1.6.3 jars.
At summary like i said: Nothing really worth to switch to 1.6.4 & lose all mods. And don't take the bug serious: This bug only happened in 1.6.3.