Let's flip the question: How lethal is your roleplaying?
For example, let's say there's a crime lord the PCs need something from. They have the choice of either attacking the lord's lair (combat) or negotiating with him (roleplay).
How often are you willing to have the PCs outright fail if they undertake the second option? And have the crime lord kill them outright.
Most DMs I've seen will "roleplay" for a while, as the PCs pull out all sort of ideas, but will let them be sucessful in the end. So if the combat option is lethal, but the roleplay option never is, then obviously players will roleplay.
But is that the correct response? Sometimes combat is the correct response. To me, a lot of the discussion in this thread states that negotiating in the scenario above is the more desired option. I'm not sure that's correct. Both options should be equally desirable, equally different, and equally lethal.