I think some RPing goes beyond the boundaries of gaming or at least make no sense to me whatsoever.
For example, i remember years ago someone on these forums posting about RPing a city guard. Now, i'm not one for knocking anyone's play style, but all he literally did was stand at a gate in Bruma until his shift finished, went and had lunch with the other off duty guards, then went to bed until his next shift. He played the game for over 200 hours doing this.
Are you serious? I litteraly laughed outload at this post. I take my roleplaying as seriously as the next roleplayer, but I cannot imagine anyone doing that for 200 hours. That's dedication. I have heard of folks in Oblivion who roleplayed mountain men that never entered the cities, hunted all day and slept in tents at night. I even tried that for a while, but it got old quick, especially because there was no one to sell my pelts to as you have to go to a City to find a merchant who will buy pelts. Skyrim has lots more opportunity for this type of roleplay, since you can find other hunters and travelling Khajiit to trade with, and you can make stuff with your pelts, not to mention mining.
But I digress. To answer the OP: Yes, I roleplay and for the same reasons mentioned in other posts. It is fun, it allows me to feel more connected to the game world (more immersion), it increases replay value, etc. etc.
Also, roleplaying lets you spend some time thinking up new builds and then seeing how they will far in the world. That is something I did a lot in Oblivion, and although the "class creation" has been eliminated in Skyrim, there is still some opportunity for creating some interesting roleplaying builds (although not as much as Oblivion and Morrowind).
Roleplaying lets you be someone else. So, if in real life you are basically a nice guy who would help a nice old lady across the street, in the game, you can roleplay a thief who pushes her in front of a carraige to create a diversion so you can pick that nobleman's pocket with his noticing.