Well, let me sound a dissenting note.

As a DM, I like wealth by level quidelines.
1. It frees me from the need to select and place treasure. I can just tell the players: You have gained a level. You now have X gp to spend on new equipment (or alternatively, in 4E: You get a new magic item of level A and B gp to spend on new equipment).
2. It cuts down on a potential source of player-DM conflict. Not that I personally need it for my group, but wealth-by-level guidelines are a neutral and objective way to answer the question: How much treasure should a PC have? While wealth-by-level guidelines have been criticized for fuelling the players' sense of entitlement, the flip side of the coin is that they also serve to moderate player expectations. Low-level players don't expect to find a
+5 holy avenger on an adventure, and they will likely not be bothering the DM to give them one - at least for a few levels.
3. I'm not in the business of rewarding my players. We are friends and equals. When I DM, I'm in charge of running the game and making sure that everyone has an enjoyable time, but I don't dole out rewards (or punishments) like a parent, teacher or supervisor.
4. It helps manintain the balance between the PCs, and between the PCs and challenges. If I ensure that characters of level X have the standard wealth that the game assumes, that is one less factor that I have to take into account when deciding what would be a suitable challenge. And characters with equal amounts of wealth are more likely to be able to contribute equally in most situations. I consider both these things to be plus points.
5. At the end of the day, guidelines are only guidelines. I can break them if I want - and I do from time to time. I just need to be a bit more careful when I do so. And as for influencing DMs one way or another, well, DMs who want to ignore the guidelines will, whether or not they are given explicit permission. DMs who always want to follow the guidelines will, even if they are given explicit permission not to. And the DMs who will only ignore the guidelines if they are given explicit permission to - how many people like that really exist, anyway?