The need for it is, in my mind, relatively simple. There is no way for a PC to have any idea how strong a monster is. NWN2 will allow for scaling and texturing of creatures, but builders don't have to use those. So a PC can encounter a group of monsters which are easily slain (werewolves) and then another similar group which beat the tar out of said PC (werepanthers in my case, though I've seen this the worst with goblins on many servers). Most of the time the PCs realize they are going to get pwned and get away, but this isn't always the case. Sometimes monsters have immobilizing powers or spells, or can simply out-run the PCs. Large crit multipliers can be nasty as well. Given the hugely-higher risk/reward ratio of most static fights vs. DMed ones, it makes sense to me to give PCs a bit of leyway here. Trust me when I say that slaughtering a tribe of CR .5 to 2 goblins only to enter the chieftan's tent and find he is CR 12 is NOT fun

You may say that being able to tell the power difference between two similar-looking creatures isn't realistic, but I don't think D&D's combat system is realistic enough for that to be much of an argument. Sure in real-life a "level 10" bandit would look more or less like a "level 1", but he also wouldn't have 100hp and be able to take a fireball in the face and laugh at you. In addition to that, there are many visible cues we can't account for without giving each creature a unique model, or without a DM present. Even if the models were life-like, there would be things only an experienced adventurer (and no the player himself) might notice, such as the specific spell foci hanging from a socerer's neck, or the curve and length of an enemy's longbow.
I'm a bit on the fence about how to do this. A while back Spider Jones mentioned a PnP mechanic to me of a sense motive check vs. bluff or intimidate, but I can't seem to find that in the DMG. I'll follow this post with the rules if I can find them, though hopefully Spider will chime in.
The good news is, this is probably something thats really easy to implement. And we can provide builders a way to modify or disable the results on the creatures they create.