Me and my friend were talking about this. He never changes the difficulty. He played the same fight over and over again on Mass Effect, because he didn't want to change the difficulty down a smooch.
I've only ever did this twice in oblivion, when I was fighting mankar Camoran, and when I was in the arena and fighting the altmer with grey aegis. Being a pure mage, these fights svcked hard. There was no way I could defeat them on default difficulty, so I turned it all the way down, killed them in one shot, then turned it back to normal for the rest of the game.
So, are you the stubborn type, or the guy who just wants to have fun?
1) It was confirmed that you can't change the difficulty, the game auto-sets it depending on how well you're doing.
2) If there is a slider, I will play two games. The first I will just do the main quest, with the difficulty to the max. I'll then do as many sidequests as I can find. The second game, I will RP the entire time with slider set to the minimum (so I don't have to care about optimization in building my character, or equipement type.)