If your campaign / game is about surviving a prehistoric jungle then you wouldn't use gold for XP. You would probably either use XP for dinosaurs, or maybe XP for every day they stay alive.The thing that always conceptually bugs me about XP for gold is that it doesn't allow for XP from something like being dropped in a prehistoric jungle and fighting your way our past a bunch of dinosaurs and such (no loot). That's something that seems to me like it should provide quite a bit of XP, whereas slipping past the guards and grabbing a pile of gold seems like it should provide minimal increase in your combat capability (and levels primarily increase your combat capability).
So, for those who are fans of XP for gold, how would you address my concern?
If the game is about exploring dungeons and recovering treasure, than recovering gold represents the culmination of an aggregate sum of activities taken (using class abilities, combat, problem solving). Levels don't only increase combat capability but also increase your character's general dungeoneering skill (improved saving throws, hit points, skills, etc... all increase your character's capability of handling a dungeon environment).
In this style game, XP for Gold is appropriate. What is the indicator of a successful dungeon delve? Returning with a grand pile of treasure. A delve that resulted in killing a 100 creepies but results in a handful of rusty swords and 25 ep would be a failure.
XP for gold works in this style campaign. It wouldn't work as well in a different style campaign.