Here's the disconnect: you seem to think that getting to 30th level has to be an achievement. In a more "casual" playstyle, it's not. If the DM and the players want to have a 30th level game, the DM can just prepare a 30th level adventure and the players can just create 30th level characters, and they can play.
Interestingly enough, this probably has a direct relationship to the problems with higher-level play. How does a designer know what challenge level is appropriate, when he doesn't know whether the players in question will have "learned the system" through 30 levels, or be (effectively) 30th level amatures?
Because, conceptually at least, "earning" XP isn't about pleasing the DM. It's about demonstrating that one has learned the game system sufficiently to handle harder challenges. Especially in earlier editions, with their flatter power curves, it was the experience of the players, far more than the experience of the characters, that often determined success or failure. The XP system provides, at least in part, some method to determine when a
player has learned enough to take on harder challenges.
Now, of course, one might say "
I don't need to demonstrate any such thing." But that same person, as often as not, is the one complaining about asshat DMs who throw too much at them.
When playing 3e, I used a 1/2 suggested XP houserule. When asked why, my answer was simple: "To give you a chance to master your powers before you gain new ones." There was some grumbling, but when I ran WLD using unmodified XP rules, at least one of the players was willing to admit that he could see my point.
There are certainly some players who can read the rulebook and then jump into the deep end without problems. IME, these players are few and far between. Players
who think that they are these players are, OTOH, a dime a dozen. Again, the XP system gives the DM a reasonable idea which type of players he actually has, and this is vital information when designing challenges.
A longstanding group might not need this benefit, of course. A sandbox campaign where the players decide what they are willing to attempt might not need it, either. Overall, though, I think an XP system is a valuable tool for most DMs, most players, and most campaigns.
One might also point out that there are more than a few players who, having survived the Tomb of Horrors, feel that their success is penalized when Bob and Joe get the same XP, the same treasure parcels, and the same levels, despite doing nothing whatsoever. Despite, perhaps, not being there at all. YMMV. IME, players who prefer that others don't get the benefits of their success are a dime a dozen, too.
Finally, one can take an XP-using game and modify it to not use XP with very little difficulty, as many in this thread have demonstrated. It is not as easy to take a non-XP game and graft an XP system onto it. IMHO, at least. Therefore, while one might cease to use the XP system to expand one's horizons, the game that has an XP system that can be used or not used as the group desires automatically allows for a wider range of playstyles than one which does not.
RC