[Steve Plambeck]

I'm considering too that (in my experience) the best possible use of gold once you have it is to spend it on fine weapons, silver weapons (for wizards), and fine armor. All of which is to give your PCs effectively higher attributes in those do-or-die situations.

XP is (or at least was) mainly spent on also increasing attributes to improve survival odds.

If I've earned enough XP to up my DX +1, or enough gold to buy a fine sword that ups my adjusted DX +1, then I've increased my effective DX +1. Very rarely I might even do both in the same adventure, allowing me to go into the next session at +2 compared to what I had been.

That's not going to be as rare with extra XP attached to the gold earned. It's going to accelerate attribute bloat, or effective attribute bloat. The bigger the treasures (and the fewer survivors to split it with, which does happen on occasion) the faster a PC might increase how powerful they are; a real killing could see a PC jump an effective 2, 3, or even 4 points in just a couple days of play time.

The intent of the rules is to slow down how fast a character advances as they become more "powerful". XP for gold would accelerate that advancement, possibly to an unbalanced degree if there was "too much" treasure to be had. How much gold is "too much"? Well, that's not something I'd ever want to worry about. PCs love to shop for new toys, and I wouldn't want to deter that.

I've had PCs that grew quite rich, and that made for some interesting and fun situations in and of itself, but they didn't turn into 50 point superheroes as an unintended consequence.