Currently there are no powers or abilities that require the player to know the exact HP of a monster, so that isn't something a DM should be forthcoming about (except in descriptive flavor perhaps).
It definitely helps when you want to decide on which power to use on it. If it still has a healthy dose of hp, then maybe you want to use a powerful ability like a daily. But if it has only a little hp left, then no point going all out. Might as well conserve your resources and use an at-will, or maybe an encounter power at most.
It is the same concept as with minions. Would a rogue bother wasting an encounter power like torturous strike on a kobold when a simple at-will will suffice, if he knew that it was just a minion with 1hp?
"He's starting to show signs of wear. He's moving a bit slower and covered with countless small wounds, all dripping ichor. Nothing too major, though -- he's got a lot of fight left in him, unlike his allies. He isn't yet bloodied."
Great narration, but I have one question. How feasible is it to expect the DM to be able to come up with all sorts of appropriate descriptions for every monster in every encounter the party faces? That was one problem I faced when I tried something along this line in my 3e games. The first few encounters worked great. But by the 4th fight, I was rapidly running out of different and ingenious ways to describe the extent someone (or something) was wounded without giving too much away. It soon became tedious and tiring to repeat myself over and over again like a cracked record, and the players quickly caught on as to what sort of status each description was referring to anyways.
It is interesting to note that in my games, hits are sometimes really direct hits, and not just misses. So you do get scenarios where fighters are literally trading blow for blow.
