How good it is depends on the actor/threat model. I'd say that server-side encryption is always a good thing when you include someone trying to steal disks from a SAN array to see if there's tasty data on it.
On the other hand, if your adversary wants to get your stuff, they will find a way. The whole cryptography thing is just imposing cost on a potential attacker, not a universal warranty against any possible attack. Someone can still locate you and beat you with XKCD's $5 wrench for your password.
Ideally we'd have end-to-end encryption on everything without adding complexity for end-users. But a lot of that stuff seems to be hard to build and at least just as hard to retroactively bolt on to a system. iMessage (iChat) goes back a long time and supports many platforms (yes, within the apple ecosystem, that is) which means they can't easily nuke every legacy API at once.