From the article about rogues I linked to I found the following about diplomacy vs bluff:

5a. Even with good threat reduction, you're going to find yourself gaining aggro periodically.

When that happens, a good diplomacy skill is very helpful to any rogue. Again, rogues get a ton of skill points and this is a good place to use them. I often read that rogues are debating taking Diplomacy versus Bluff. It's not even close. Bluff is a very flawed skill and doesn't work nearly as well as advertised (it may actually decrease DPS) and Diplomacy works extremely well. It also uses the same stat (charisma) as UMD so you can get a very high skill with little effort.