I've seen many roleplayers use a blatantly unfair tactic when they roleplay, and I think that some of them didn't even realize what they were doing. I call this tactic selective advocation. Here's how it works; a player advocates something such as simplicity or realism when it benefits him or her to do so, but not when it works against him or her to do so. Players might want every unrealistic rule that benefits them to be followed to the letter, but loudly complain about every unrealistic rule that hurts them. They might complain every time the GM simplifies the rules in a way that hurts them, but cheer every time the GM simplifies the rules in a way that helps them.
A classic example is the way players often react to called shots (attacks aimed at a specific part of the target's body.) Many GMs do not have NPC foes use called shots, in which case they are something for the players. Often players are perfectly fine with an unrealistic hit point system for handling wounds, but when they target a specific part of a foe's body, they expect things to suddenly become realistic and have the attack effect the target like it would in real life. Actually, they sometimes expect the attack to be even more devastating that in real life (for example, they expect a successful called shot to the head to always cause instant death, even though not all head hits are fatal in real life.)
In conclusion, selective advocation is unfair and leads to overly easy games that don't test your skills and don't give you a chance to improve them. As I said before though, many people who use selective advocaiton do not realize what they are doing. Don't yell at them for it. Instead, calmly explain it as a friend.