You realize, that at least in d20, those wealth by level guidelines are not entitlements don't you?
They are guidelines for starting equipment for making characters who start at that level, not for characters who have had years of adventuring experience starting at level one.
I think you misread the original post. Those WbL guidelines are often considered "entitlements" by players, and they become a straitjacket for many GMs. I know this all too well, because I felt I was in that straitjacket when I played 3e. And the current treasure packet system is even worse - it is probably my biggest beef with the 4e rules.
In other words, if a character reaches level five he does not automatically get all that gold or items. They do get it if they start at level five.
In other words, if the GM doesn't follow the guidelines, one of two things happen:
1) You'd be better off making a new character at level 5, because your current level 5 character is "undertreasured". In fact, this is a good option even if your current character is close to his expected wealth... because when you make your new character, you can customize your gear to fit your new character!
2) You make a character that is now under-equipped to the rest of the group, and unable to swing at the same level as everyone else. Which means that when you die, the rest of the group gets your gear.... meaning they have even more wealth than you do the next time you create a PC.
In other words you are not guaranteed to get that +5 weapon at level 25 unless you started at level 25.
So, if I want that +5 weapon, I better make a new PC when I hit level 25. To hell with that guy I've been playing for two or more years. I want my holy avenger!
They are also a way to help the DM to determine what personal items an NPC could have as well.
Which is fine. The problem is, that 5th level aristocrat should probably have a lot of money, and not much in the way of items. That 5th level warrior should probably have a lot less. Those rules, as written, don't differentiate. These guidelines work great as guidelines.... but they are often taken as an extension of the rules, and that creates problems.
The guildelines were written with the highest of intentions, but they backfired and were considered by MANY to be de facto rules. This is a common complaint among adventure writers - if they don't follow the WbL guidelines, they don't get printed, or if they do get printed, the first complaint fired against them is that they don't follow the "rules".
Which leads to every 5th level dungeon having the same degree of treasure. And all those fun items getting dumped by players who need to fill their big six slots with boring items to stay in the "monster curve".
Because let's face it, a player who's had more experience from level 1 is more than likely to have more money that the level guidelines, and possibly even better stuff that's worth more that the level guidelines.
Not true, at least in my experiences with D&D. The only way this happens is if the GM is giving out awards in line with his players' "wish lists", and only that, or if the PCs are always selling gear to buy their dream items, and are clever with their acquisition and retention of treasure.
If the GM hands out interesting magic items that don't fit one of the big six slots (Amulets of Protection/Natural Armour, Rings of Protection, Magic Weapon, Magic Shield, Magic Armour, and Belts/Girdles), then PCs fall behind on the curve. In other words, if you give out a lot of magic carpets, decanters of endless water, cubic gates, ioun stones, figurines of wondrous power, and the like, and less +X items, your players will fall behind.
And in those cases, the new guys, who got to choose their own stuff, are going to come out a helluva lot better. I've seen this happen many, MANY times. In one 3e adventure path I ran, at around 6th level all of the PCs except one were swapped out, partially so the players could cherry-pick their new items.
I'm sure there are playstyles that can ignore this in actual play, and if you've found yourself in one of those playstyles, you're a lucky guy. Unfortunately, not everyone finds themselves in that boat, and WbL guidelines can reward all sorts of play that is really not conducive to long-term, immersive ("simulationist" style) campaign play.