Heh... if deviating works, not deviating probably also works
So if we accept that following the guidelines or wildly deviating from them both work, the guidelines themselves are not particularly helpful. They only have worth if it's clear that following them is better than ignoring them.
Anyway, by "it works" I just meant that mechanically I've never noticed any problem with using 3e guidelines for wealth by levels, but the problem for me was that it forces a specific fantasy genre, with relation to magic items commonality.
That's fair enough, but I think there are an abundance of clear mechanical problems. For one thing, it's hard to justify a growth rate as slow as it is. For example, let's say my 5th level character has 9000 gp worth of stuff. By default assumptions, he's in a party of four, and levels up when that party defeats the equivalent of 13 standard level 5 characters. If all of those enemies have treasure equivalent to the PCs, the PCs' wealth just quadrupled. Even if you assume that some enemies do not have treasure, or some is destroyed or useless, or even if you follow the ludicrous guideline that says NPCs have less wealth than PCs for some reason, their wealth should still probably double or so every level. Which it doesn't. And that's assuming that killing enemies is the only way PCs acquire wealth. The guidelines themselves are really very difficult to support.
Conversely, you have an economy set up for goods and services, and magic items completely break that economy, as has been covered elsewhere, by being ludicrously expensive but implicitly not all that rare.
Once I became tired of that genre, and wanted to run adventures in a more magic-item-scarce setting, deviating downward wasn't that easy for me, I couldn't just use lower level monsters and be confident on the resulting balance because there were things that scaled with level and things that scaled with equipment (but were in-built with monsters of that level). So for us it actually worked well, as long as I did follow those guidelines, but this restricted the fantasy genre somewhat. The best idea I could come up with, was to start "merging" the abilities of multiple magic items together (even tho technically by that system the value of such item would be more that the sum of its part) in order to at least reduce the overall number of items, without changing to total sets of benefits.
And that's an issue as well. In D&D, a large portion of a medium to high level character's combat prowess is determined by magic items, especially defense. Using the wealth by level guidelines, combat characters are gimped. High-level characters get to the point where they can't miss even good defenses for their level. If you want a low magic campaign, this problem becomes worse. Spellcasters ironically have virtually no use for magic items, and can create their own for cheap if they like, while martial characters become incompetent fools without their special gear.
Conversely, if you do what I do, the opposite, and indiscriminately give characters huge piles of treasure, the martial characters become more effective, defense rules, and everyone is decked out like a Christmas tree. This "works", but certainly not for everyone.