I forget when and where, but I read a nice little article on this subject not long ago. Its thesis was that many D&D players and campaigns rely heavily on combat to solve problems because their combat skills are well-defined and reliable, while their social abilities were not and to some extent always depended on DM fiat. For example, a rogue needing to escape an area and make their way past a guard could choose to bluff or hide, and sneak attack. If they bluff and are forced to role-play and await the DMs judgement on how believable their lie was, then the outcome is highly uncertain from the player's point of view. On the other hand, if they sneak attack the player has an excellent idea of what their chances are of taking out the guard (and they're probably quite high). If the player wishes to survive they probably choose the sneak attack, and similar arguments can be made for choosing a combat solution over any other social interaction.
If, on the other hand, they know that this kind of bluff has a DC of 15 and they've maxxed out their bluff ranks, and can trust the DM to acknowledge their investment by calling for a roll regardless of their role-play effort (maybe they just aren't feeling very creative that night, or the player might just be lousy with words) and abiding by the results, then bluffing becomes an equally attractive option.
In short, if they players trust that investing points in social skills will yield reliable in-game results then they will use those skills to good effect. Calling for skill rolls in challenging social situations actually encourages role-playing, because it encourages players to invest in and use these skills.
--Ben