The rules for intimidate seem strange to me. I don't understand why you get a -10 for dealing with hostiles. I understand that without that modifier it would be overpowered, so I would say that the DCs for Intimidate should be arbitrarily higher overall (except for monstrous humanoids who might respond better to intimidation as it's a bigger part of their culture than diplomacy is.)
However, I think it should be just as high for using it on friendlies, too. I'm no psychologist, but--speaking personally--if someone I perceive as friendly tries to "intimidate" me, it's much less likely to work than if they "use diplomacy" (or bluff, if I don't catch it!) It catches me off guard and puts me on the defensive... and doesn't make me especially more likely to follow along with what they want. If someone who I already expect to be hostile/intimidating does so, then it won't necessarily work better, but it won't work worse either.