Why do you prefer playing PR instead of vanilla BF?
Teamwork.
In arcade FPS made for the mass consumer, the gameplay cannot be teamwork oriented because most people are not. The result is that all weapons are not as powerful as they should be.
For example, in PR the heavy AT is devastating to armor, but in order to use it well you'll need the support of your squad (lone wolfing will get you nowhere). In vBF2, they can't make a heavy AT as powerful as it should be because they can't inject the teamwork element. So they nerf down the AT role so a player can lone wolf it, get like 6 rockets, but not be as powerful.
When I played vanilla BF2, I would create a squad and invite a bunch of people into it. Then as squad leader I would hide and let squad members spawn on me. I wouldn't do much fighting... I would just lay around in hiding and watch my squad mates spawn on me and take the flag. I got fun out of changing the tide of the battle even though I didn't fire a single shot. I also knew that my efforts made the game more enjoyable for my squad mates.
That aspect of teamwork is what I love most about PR. Sure we could spent 15 minutes just standing around sometimes defending a flag or spent 10 minutes just driving to an objective. Last night I got up, went to the bathroom, got a smoke, got another beer and when I came back we were still driving to an objective on Kashan

I actually enjoy this. It makes the reward of winning so much sweeter.
One thing I also noticed is that I appreciate the game engine more. Anyone else feel this way? In vanilla BF2 I would spend so much time racing for the next flag that I didn't have time to appreciate the little details in the game world. In PR, you learn this quickly when a wooden box that you otherwise would have never noticed saves your life when crouch behind it.
Do you prefer more realism in a game?
Yes.
Can a game have to much realism? If so: at what point does a game have to much realism?(Like when vehicles needs petrol, avatars need to eat, go to the toilet or other stuff like when shot in a leg you can't use that limb etc.)
This is a difficult question that I cannot answer. Everyone has a personal preference (as you can see by some of the other answers). All players have a different threshold for realism.
In fact, there are some players that don't like certain aspects of realism in PR, but they deal with it because the overall experience is awesome.
I think what matters most is the overall game experience.
When does a realistic game become a simulation instead?
When *NOTHING* is automated. Every single task from looking, to walking, to using your arms, etc is a manual action.
Would you consider PR a simulation?
No.