A bit of both.
I'm not a very diplomatic kind of guy. I don't like to use tact as I find it absolutely useless.
As a result, I'm not very able to to roleplay a character with a high charisma or with shining diplomatic skills.
That's why you do sometimes need to rollplay. That's what manu skills are for.
Take, for example, if you want to play a medical doctor. How can you truly play a medical doctor if you don't know things like terminology and procedure? That's where rollplay comes in.
So you see, there is a need for some rollplay. Not everybody is a good rollplayer. And because of that, if there comes a situation in which some diplomacy is needed, then there needs to be a roll for it. Even more so if it's a plot point that needs it to succeed in order to progress the story.