XP for gold has a venerable history, and is a totally workable mechanic.
What bothers me is some of the justifications I've seen for it.
As I see it, XP for gold is an abstraction specifically suited for adventurers, and the idea is that if you are recovering treasure, you are encountering danger, and therefore learning. More treasure usually means more danger. Fair enough.
The mechanic stops making sense if you try to extend this mechanic to non-adventurers. Some lordly Baron sitting in his keep is in no danger and is learning nothing about being a fighter by sending out tax collectors to collect money from his tenants.
Where things break down is when people start trying to find some kind of further real-world justification for it. Some, for instance, consider XP for gold to represent reputation and confidence. I find this explanation to be indefensible. If this were true, you could level up by bragging, or by drinking whiskey. The fact that you can't become a better wizard by partaking of a little Dutch Courage exposes the internal inconsistency of such an explanation.
Fact is, neither confidence nor reputation correlate with ability. I remember a martial arts class - we'd drilled defending against dagger attacks for a couple hours, and I was getting a good feel for it, and felt really confident in my abilities.
Then we tried some free-play, and in my first match, I went up against one of the most experienced guys at the salle. Well, long story short, my skull would have looked like hamburger if we'd been using real daggers. My new-found confidence helped *exactly zero* against a skilled opponent.
Similarly, even if I have a reputation far and wide as the fastest draw in the West, that won't help me one whit if that whole reputation is built on bravado, and the exaggeration of one sloppy victory. When push comes to shove, my reputation counts for *exactly zero* in a real duel against a skilled opponent.
Long story short: XP for gold good, XP = confidence and reputation bad.