Well, there is experience for defeating foes, solving "problems" and achieving certain milestones (finishing "big quests" and that kind of thing) But yes, the gold-for-experience thing comes down to blowing it. However, that spending can take many forms (as listed above).
“Let me live deep while I live; let me know the rich juices of red meat & stinging wine on my palate, the hot embrace of white arms, the mad exultation of battle when the blue blades flame crimson, and I am content"......Conan the Cimmerian.”
― Robert E. Howard, Conan the Barbarian
The main idea is that you learn more by putting yourself at risk, living large and drinking deeply from the wellspring of life. While people who play it safe are the kinds of responsible folks that never leave home, choosing instead to live nice quiet lives as 0th-level farmers and midden cleaners. Mechanically, this has the advantage of giving you the opportunity to choose how fast you level up: Either you risk a lot and possibly put yourself on the verge of poverty or you take some of that money and invest it on better equipment, spell research, henchmen, training, (which allows you to re-roll your Hit Dice at level up) or even a stronghold when you get to high enough level - the difference here is that you get experience when you spend on things that don't directly benefit you mechanically or at least incurs a big risk.
And lest anybody say I am not a merciful god, If you choose to use the carousing table (
Here) you get double the experience points vs. just telling me that you spent a certain amount on hookers and blow!
Bottom line: Choices and consequences ... on the other hand, if everybody hates this idea, then let me know. Just remember that I plan to give you ample opportunity to acquire the means to support yourselves (or die trying). Speaking of XP, everybody earned some from that fight ...