Ignore the WBL Chart at your own risk
Especially if you’re going under.
The WBL tables are a suggestion, but changing WBL and ignoring the tables is an extremely dangerous thing to do within the mathematics of 3.x. Few things are as broad in their scope. Changing wealth affects every character in the world, or at least every player character, and can have some very far-reaching and unexpected side-effects.
In particular, the system responds extremely poorly to lower-than-normal wealth. The WBL provided in the Dungeon Master’s Guide is very close to the bare minimum for 3.x to work as expected without massive changes.
Make no mistake: wealth is effectively non-casters’ access to magic, and magic trumps everything else in the system. Starving those classes, already the weakest in the system, of their poor access to magic results in severely exacerbating the existing imbalances within 3.x.
As a result, lowering wealth should only be done when you are also making sweeping changes to the system in general. Reduce access to magic (including through class features), and eliminate the (many) encounters that under normal circumstances require magic to solve. Be prepared for a lot of detailed work converting everything. It is not something to be done lightly.
Wealth above the guidelines is easier but can also be dangerous. Because magic is so powerful, more wealth means more magic, which means characters can pull out “trump cards” more often. At extremely high levels of wealth, all characters are effectively full-casters, since they’re all just using magic items to replicate the spellcasters’ native magics.
Judging Parties and Encounters
Anyone who has looked through any significant number of monsters in 3.x with a critical eye can tell you that monsters’ CR is so frequently inaccurate as to make the entire system near useless. As a result, DMing always requires judging the actual abilities of parties and encounters, not simply blind adherence to CR and EL rules. Following the Wealth by Level guidelines does not guarantee that any given party will be able to handle any given CR-appropriate encounter.
The problem is magic has the ability to trivialize encounters. The right spell, and by extension, the right magic item, can eliminate certain threats. And the lack thereof can make what should be merely an obstacle into a death trap.
And magic is not equally available to all classes.
A mundane warrior literally cannot fight a Ghost if he doesn’t have a magic weapon. His abilities have a 0% chance of doing anything at all to Incorporeal enemies. A Cleric can just Turn/Rebuke it, a Wizard can command undead, and so on.
Meanwhile, it becomes extremely difficult to find a mirror example, where the spellcasters cannot do anything but the warriors can. Most examples involve heavy DM-fiat, such as permanent, expansive, and arbitrary Dead Magic Zones. Antimagic field is high-level and has a tiny area. Golems aren’t really immune to magic, they just have SR ∞, and competent spellcasters prepare ways to handle SR. And so on. Any problem you can come up with, magic has an answer to.
What this winds up causing is making an already-serious problem even worse: it becomes very difficult to challenge spellcasters without sidelining mundanes, and even harder to give mundanes appropriate challenges without some arbitrary excuse why the spellcasters are unable to just magic it away.
So while a very experienced DM, with a group of very experienced players, who have played together for a time and who are all on the same page in terms of expectations and how to accomplish them, can afford to simply ignore the Wealth by Level guidelines, any kind of heterogeneous group risks having an extremely serious problem where players cannot meaningfully contribute to the same encounters.