While this does save a bit of screen space, it is frustrating when someone replies with "I'm running behind, if you don't see me in 15 minutes just go ahead without me." and you didn't notice the message right away. Apple deserves a smack-in-the-back-of-the-head for this one.
Fortunately, this is resolved on Android. Every message shows the time stamp. Comparison shots below.
(iPhone screen shot courtesy of tyler ingram's blog)