Does diplomacy, bluff, and intimidate have any real purpose in the game? I'm almost done it and i haven't found much of a difference.

<font color="gold">It sure made a difference for my Paladin. I don't have any clear examples, but I know that those abilities are implemented into the game. ;) </font>

Yes, they affect which dialogue options are available. Also the results of some quests are different (rewards).

But dont waste your time with them if you are not a paladin or bard.

Diplomacy: Good NPC's will be a LOT easier to talk to to get information (even some quest realted) and thank you more (rewards). Intimidation/bluff: Evil NPC's will spill a lot of information before you kill them and possibly loose morale quicker in battle. Being skilled in diplomacy might even help you avoid confrontation (In RPG's, that is...)
I think all of these skills will get you better prices in shops as well.

However they're not essential, but adds variety to the storyline. So if you're a "power-gamer" forget about it.

Other factors are: High intelligence and wisdom scores will also give you more dialogue options. High charisma will make people approach you in a less hostile manner and reduce prices.

Clear examples: None that I can remember - I might just have made my entire reply redundant here:)