Normally Diplomacy doesn't force them to act friendly toward you, you genuinely get them to open up and increase their attitude. People who are friendlier are more likely to want to help. Example would be, "I'm a good guy and I'm here to help, let me know where the watch captain is, its important."
Bluff doesn't exactly force them to act friendly to you, but you can use it to trick them so that they do provide you information as you were their friend. Example would be, "I am the watch-captain's best friend, have you seen him on duty tonight?"
Intimidate makes people act nice toward you, at the cost of friendliness later. Example, "Give me the location of the captain or I'll find your son and that won't be pretty."
Success: If successful, the opponent will:...give you information you desire
...take actions that do not endanger it
...offer other limited assistance
After the intimidate expires, the target treats you as unfriendly and may report you to local authorities.
Basically it allows you to use your Bluff or Diplomacy in place of Intimidate to do those effects (such as getting temporary friendliness).
Its not that great with how many Skill Points Rogues get each level.
IMO.