Lots of good stuff in this thread. I agree with most of what pawsplay and Saeviomagy have been saying.
Another point: The NPC Attitude chart is basically a tool for the DM. It allows a DM to quickly determine how an NPC will react to the PCs on the fly. In order to keep the DM from having to create dozens of complex psych profiles, it simplifies an individual's attitude into five quick and easy categories. Either an NPC is "Helpful" or he's "Unfriendly." Diplomacy adjustments are made, and then the DM determines how the NPC acts, based on his attitude.
But PCs are far more than NPCs. They are as complex as the real people playing them. You cannot simply determine that a PC is "Friendly." A barbarian might be "friendly" towards the party sorceress simply because he wants to get into her pants. Or, he might be "friendly" because she reminds him of his long-dead sister, whom he loved very much.
Now the sorceress bats her eyelashes at the barbarian and charismatically tells him to carry her over a puddle. Well, if the barbarian is really just hot for the sorceress, he'll be eager to do it. He'll become "helpful." But what if it's something else? What if he is suddenly reminded by this vain and selfish request that this sorceress is NOT in fact his sister, who was a selfless and independent woman? The realization is jarring, and makes him angry. He becomes "hostile."
PCs can't be treated by the same shorthand attitude rules as the NPCs, because they are more than the throw-away type NPCs that the attitude rules were meant to serve. They can be spontaneous, contrary, and completely without reason. Only the player knows for sure.
It's for this reason that diplomacy isn't supposed to work on player characters - because this is, in the end, a game.
This is also a very valid point.