Jonesy89: Because certain approaches are more rewarding to character growth than killing. To grossly misquote Casino Royale, "any PC can kill at the first sign of difficulty", but taking alternative paths inherently demand more of the character in terms of restraint and creative thinking. If experience is supposed to represent what the character has learned (which is the way XP is explained in P&P), I can't see a character learning anything or evolving nearly as fast if all they do is act like a thug.
eVinceW21: To use your Casino Royale example, Craig's Bond is portrayed as a dumb thug. He is not the witty creative iterations of Bond that have come before, yet he still gets the job done and he is still a proficient killer.
Except for the part where Bond spent over half of the film playing cards with the villain in order to bankrupt him; sure, Bond could have just blown his brains out with a high powered rifle from a mile away, but he had to restrain his violent urges as much as possible to not endanger the plan, otherwise they would have lost the opportunity to get LeChifre to defect and provide valuable intel. Granted, he died anyway, but it's not as if Bond could have known that.
eVinceW21: True, but there is a discrepancy between what the player is capable of and what the character is capable of. Statistical character growth measure's character proficiency. Even if the player doesn't unlock new combat moves or techniques, it can still be said that the character is becoming more proficient at what he does. Hence, as Nameless swings a hammer he becomes better at it. How exactly he becomes better at it doesn't necessarily have to be defined, but one could naturally assume that he learns feints and parries, blocks and other more expert fighting techniques even if the player is not made aware of them. Same can be said for learning how to be a smooth talker. Figuring out how to read facial expressions or perfecting ones poker face. Or breaking locks. Sneaking around. Why should the stealth or diplomacy approach be considered preferable to a violent approach? Just because violence is bad doesn't mean players should be penalized for fighting. I can be just as much of a jerk with words as I can with my fist anyway. And if I want to play a jerk I should be able to.
But those moves only serve to kill. There is no option to disable but leave alive, to hurt with intent to intimidate and then stop to give the opponent time to yield, no option to ride the rope of a chandelier, no option to bring down the ceiling to incapacitate that golem noone in the party can hit. All combat does is result in the same tired outcome, namely hitting the dude until he dies. It's a simple, blunt approach, one which requires little to no intelligent thought on the part of the character. Were there some way for the PC to demonstrate their character's intelligence in combat, I might get doling out more XP, but as it stands fighting involves no thought beyond what is required of any character in order to kill. Simply put, characters who act with limited intelligence level slower because their simpler tactics do not result in them developing exceptionally, and as a result they do not learn as much as other characters; ok, leveling makes them better at hitting things, but that's the standard for every character.
tl;dr: just fighting is simple and results in characters learning nothing but the bare minimum, barring the character using their combat skills in an intelligent manner that demonstrates that they are learning how to use their skills in new and creative ways. That's not to say playing a pure fighter who never goes beyond hitting things with sticks won't level appropriately in some games (like, say, mine), but their contributions to the party XP stand to be smaller depending on their actions.
