Well, they make it a VASTLY different game than the BECM or 2e that I played before. As DM I run worlds that wouldn't have them, and as a player I generally avoid using them even if they exist.
There's a great deal of logic lacking in the demographics and community wealth guidelines. For example, you need a 12th-level caster with the right feat to make a Ring of Protection +1. Large towns are not expected to have such casters. Yet, EVERY large town is expected to have Rings of Protection +1 available for sale.
They also make the "recommended wealth by level" guidelines rather useless. Consider 4 PCs who each find 10,000 gp worth of magic items that are not specifically optimized to their builds.
PC #1 is old-school. He makes the best use of them he can, or gives them to cohorts or followers. Buying and selling magic items is not the stuff of heroic adventure.
PC #2 works under the 3e default assumptions. He sells these things for one-half their value and buys 5,000 of items tailored for him.
PC #3 somehow sells the items at full price and buys 10,000 gp of items tailored to his needs.
PC #4 is rather self-reliant. He sells the items for 5,000 gp, invests 400 XP, and crafts 10,000 gp of items tailored for him.
Should the DM change the allocation of treasure among these PCs? Do the wealth by level guidelines recognize that 10,000 of customized and chosen items are far more valuable than 10,000 of random magic items? Do they recognize that a PC with crafting feats may have more GP value in gear at the cost of those feats?