From the AD&D DMG, p 85:shouldn't PCs get XP based on their class? Warriors kill, thieves steal, wizards learn, and priests proselytize.
Players who balk at equating gold pieces to experience points should be gently but firmly reminded that in a game certain compromises must be made. While it is more "realistic" for clerics to study holy writings, pray, chant, practice self-discipline, etc. to gain experience, it would not make a playable game roll along. Similarly, fighters should be exercising, riding, smiting pelts, tilting at the lists, and engaging in weapons practice of various sorts to gain real expertise (experience); magic-users should be
deciphering old scrolls, searching ancient tomes, experimenting alchemically, and so forth; while thieves should spend their off-hours honing their skills, "casing" various buildings, watching potential victims, and carefully planning their next "job". All very realistic but conducive to non-game boredom!
deciphering old scrolls, searching ancient tomes, experimenting alchemically, and so forth; while thieves should spend their off-hours honing their skills, "casing" various buildings, watching potential victims, and carefully planning their next "job". All very realistic but conducive to non-game boredom!
From this we can work out what, in Gygax's view, made for an interesting and playable game - namely, raiding dungeons trying to extract gold.
More generally, if gaining levels is meant to be a significant element of the game, then it makes sense that the trigger for gaining levels should be doing whatever it is that is worthwhile in playing the game.
For Gygax, that was getting loot. (And if you were unlucky or unskilled at getting loot, well, you didn't level - AD&D was meant to be a game of skill.)
In 4e, as long as you're engaging the game (via combat encounters, non-combat encounters or free roleplaying) you earn XP, at a largely constant rate (of approx one level per 10 to 12 hours of engaged play) - this is why that system lends itself well to "milestone" or "per session" levelling instead of XP.
XP-for- combat makes sense if you think that what is worthwhile in playing the game is having fights; but not otherwise.