My basic opinion is, "whatever works for your game." XP-for-gold is what I use in my games, for most of the reasons that Raggi has already enumerated. I'm more open to the XP-for-Exploration idea than he claims to be, but I agree with his opinions on pretty much all the other systems he discusses, and I actually think he's missed a point against XP-for-roleplaying, in that it can create messes, socially, for me as the DM to be grading my players like that. (Or I should say, has created messes, socially, for me.) All the systems he's discussed, and really all XP systems, are to some degree arbitrary; at the very least, the DM creates the environment that the experience is being awarded in. But I like having the illusion of impartiality, and having something other than "Well . . . because I said so," when I get asked why Player X got more experience than Player Y.
Anyway. The thing about XP-for-gold is that it tends to break down if the players decide they want to ditch the dungeon for a soap opera, or otherwise find a way to take the game in a new direction. And while I'm moving into hypotheticals here, based on my experience, observation, and general sense of how the game ought to be run, in a long-running old school kind of game, the players are going to eventually figure out a way to take the game in some kind of new direction, away from "pure" dungeon-crawling. They're going to find something that they like better than gathering treasure. Sometimes, maybe even most of the time, this is going to be something that's still largely compatible with XP-for-gold. Sometimes, though, it won't be. (Of course, as Raggi points out, this can be a great thing; having the players choose between level advancement and some other goals can be interesting, at least on an occasional basis.)
If you won't to know what to do at that point, talk to Trollsmyth. I still don't think he's told me yet what he's doing with the XP in our now heavily soap-opera-ified solo game. My character hasn't gotten any treasure in about twenty sessions. She doesn't care, and neither do I, but not every player is as nonchalant as I am about leveling, even if they are having crazy amounts of fun gossiping about boys with rakshasa and clerics or what have you.
I can, however, comment that the process itself is a very excellent thing to happen in a campaign, even if it does cause a few problems if things go too far afield: the campaign becomes uniquely yours. This is certainly achievable by talking things out with your players ahead of time and designing such a personalized campaign from the get-go, but that requires that you and your players both know what you want before the campaign starts. The beauty of a letting a game evolve like this is that you don't have to know. You just have to be paying attention.
If you've been following Grognardia's Dwimmermount posts you'll have an excellent idea of what I'm talking about--and at least an inkling of what all this has to do with XP-for-gold. His campaign hasn't suffered as extreme a drift as the Labyrinth Lord solo, which makes it a better example; I suspect its evolution is more typical for such games. But the basic pattern remains the same: it started out as a a game exclusively about Dwimmermount, guided by the rhythms of dungeon exploration. In the past few months, the characters have started to deal with potentially treacherous NPCs, uncover the schemes of ancient cults, lead their cities in response to undead hordes, and even delve a bit into some fantasy theology. It's still largely compatible with the dungeon delve, and by extension, XP-for-gold, but it's become a distinctly different game from the one the one Maliszewski started about a year ago.
And XP-for-gold was one reason why it, along with the solo game, was able to shift in such a way. XP-for-gold supports a player-driven game. It's not the only system to do so, and it's not required to run a player-driven game, but it does help: it helps the players answer that eternal question, "What do you do?" This is especially true early in the game before they've figured out their characters, established their own motives, and gotten involved in the larger schemes of the milieu. It's provides a way to run a foundation for a player-driven game before the players, and the DM, know what really interests them.
Allowing the players to drive at least the direction of the action is crucial to this kind of evolution, particularly if you want to start out simple and get more sophisticated like this. The players need to have the freedom to say, "Ooh, that looks fun, let's go do more of that." But giving players freedom can be a great way to get some very confused players, so sometimes, just to help them get their feet on the ground, you want a big blinking neon sign that says "Adventure Here!" XP-for-gold puts that neon sign right on top of any place that has a little treasure in it.
I keep using XP-for-gold as my example here, but it's not the only system that provides that effect. Pretty anything that makes it obvious to the players what they need to do in-game to get that XP will do--which means that XP-for-roleplaying and XP-for-quests won't. Whatever their other charms, such systems either don't significantly guide in-game decisions (which in other contexts may in fact be one of the main advantages for XP-for-roleplaying and similar systems, but here it defeats the entire point), or they make it very difficult to define the rewards involved ahead of time without pre-framing the decisions the players can make. Sure, a good XP-for-quest-type system will allow the player to negotiate with the DM to define what they want the quests to be, but that doesn't give them much help in answering that question. Which is great once the campaign has gotten up and running and they have some ideas already, but it doesn't do a great job as a neon sign for the early game.
There are, however, other systems that do have the right set of features, XP-for-monsters and XP-for-exploration being the most well-known at the moment. They're not quite as flexible as XP-for-gold, suggesting a more specific range of activities, so they tend to be supplemental rather than comprehensive systems, but that's perfectly fine. In fact, I consider the D&D XP system's ability to accommodate expansions of various kinds one of its strengths.
(As an aside--while I tend to agree with Raggi that monsters really ought to be obstacles to XP-collection rather than an avenue for it, in a world where most monsters are unique (or if you could only get XP for a kind of monster the first time you killed it) XP-for-monsters could form the foundation of a really interesting game that revolved around researching, tracking down, and defeating various beasts.)
Eventually, killing those monsters or exploring those locations will lead to the PCs stumbling across some larger mystery, or they'll cross paths with the wrong people during their pursuit of gold, or they'll just run into some other activity that catches their interest. And that's all to the good, because the point of this kind of experience system isn't to define the game but to provide a foundation for it. It gives the PCs something to do and a reason to get involved in the world while you're figuring out what you want to do with a campaign.