I agree, Bluff and diplomacy can't cause someone to go against their better judgement. But I use bluff and diplomacy and Intimidate between the PCs in my campaign. It can be easily abused, but I see no reason you shoeuld contnue playing with someone as immatuer that he'll take your Greatsword. Besides, remember that no matter how friendly you are to someone, you're not an idiot:
"King Arthur, I'm your lost heir and the chosen bearer of Excalibur. Please hand it over." roll Dipo and Bluff.....50+ in both.
"Sure buddy, you're my heir but unless God tell me to hand over this sword, it's staying in my hands."
Or to bring it into context in a game I used to play. Not even charm person is so powerful. My rogue was the bearer of the holy secret Mcguffin. He fell in the grasp of the Archmage who was lusting for it. He of course failed his will save (multiclass fighter rogues

) and was extremely helpful to the Archmage. But if you're the sole bearer of the key to ultimaste power and you'er also an insanely greedy bastard, I don't care how friendly you are with someone you won't hand over the Mcguffin. MY DM had no problem with my evasiveness in answering questions until the wizard became threatening which broke the spell.
Basically, if you use common sense, inter-PC skill checks are very cool. But you have to limit Metagaming. And if the Bard Villain rolled a +50 diplomacy check, you don't have to aid him, but it should make you less likely to coup de grace him. Maybe you want to save him, bring him to the light, etc... Even if you know that slaying him and then looting him is the "Optimum " choice
