I personally think that aqua and "marble" look just fine. My problem is the mix of two that we have now.
Just two?
The problem is that they keep sneaking in new sub-looks at a faster pace than any of the looks are able to catch on.
First there was Aqua. Like it or hate it but it was 100% consistent across the entire system in 10.0.
Then came the brushed aluminum look, originally specified to be used only for applications that interface directly with external hardware, started to pop up in lots of different places where it wasn't supposed to have been (Safari etc). Then the pinstripes were quietly retired. And while these 2-3 different looks were competing for systemwide domination, they introduced a new look for Mail - this was the first 'slate' UI with plain grey gradients. Ooooh said everyone, let's go with that, and applied it to Safari, Finder, iTunes etc. Then more and more iPhone-inspired graphics with light text on dark background started to show up in iTunes, iPhoto and other places, and now the white-on-translucent black has caught on and become a staple of anything related to the Dock. All these looks accumulated over the years co-exist. Buttons, scrollbars and cursors are still oldskool Aqua, and then there are older apps like Calculator and iSync still retaining the retired brushed aluminum look, and the rest are variations and generations on Tiger's Mail. Then there's the Garageband/Logic look with an audacious amount of dark grey.
Anyone reading Apple's UI guidelines and then looking at the actual UI must feel like when their daddy said "don't ever start smoking, son" while puffing away on a Marlboro.
In a way I think it's these residual Aqua elements that leads everyone to take liberties outside the UI guidelines, because they communicate that Apple have just thrown their hands in the air and given up any ambitions of unifying the interface. If they don't mind leaving something that old in there for the world to see, then anything goes. The iPhone is completely post-Aqua and it's looking just fine, with a consistent look that's instantly recognizable as Apple's, without the glossy blue pill buttons and scrollbars. Why not just go with something like that?