Celebrim, you make a really good case. But I think the rule is written as is basically for two reasons:
1. The players have created the PCs' personas. It's their call when the hero hesitates, or his legs turn to jelly. As you rightly said, the DM shouldn't be running the PCs, the players should. If the DM wants to literally control the PCs, he has to use magic.
2. The DM doesn't have time to create detailed personas for every NPC in the game. He won't know whether the shopkeeper would or wouldn't be intimidated by a particular threat. Moreover, the player may not be adept at
making a threat. So the shorthand has the PC rolling Intimidate to see whether or not his threat works. But when it's PC vs. PC, then the player gets to interpret the other player's threat himself. Granted, this is disadvantageous to the player who's making the threat if he's bad at it, but it's better than a player putting all his points into Intimidate and controlling the rest of his party with die rolls.
As you say, the DM can Bluff, and he can Intimidate, but in both cases you're presenting information or attitude, and then allowing the player to assess it. I could tell a player that the giant is so overpowering that his knees feel weak, but it's crossing the line rules-wise (and I think rightly so,) to roll an Intimidate check and--if successful--telling the player that his PC
can't move or attack.
Likewise with Bluff, you could have an NPC tell the PC that he can sell him the Brooklyn Bridge for cheap, but even with a +100 to Bluff, it should be the player's decision as to whether his PC falls for it, not the dice.
Bluff and Intimidate are nice, but they aren't equivalent to
Suggestion or
Hold Person, after all.
