Look, I am not dissing webkit devs whatsoever. They're smart people doing a great job, and they're getting strong support from their employers because doing a great job really matters. But if we end up in a webkit monoculture, I imagine much of both the intrinsic and extrinsic motivation will disappear. You're running a for profit corporation. Should you continue pouring resources into a game you've already won, or should you shift them towards something else that's going to impact your bottom line? That, and only that, is the connection I would make with IE6.
If that's talking shit, then fine I'm talking shit. But I don't think it's unfair to expect webkit's caretakers to behave like rational humans.
As for needing multiple implementations to find problems, I won't challenge the assertion that people are uncovering and resolving ambiguities on discussion forums, without needing multiple implementations. But how many are found this way? Surely you would agree that some problems are found by trying something out, having it not work, testing it in a different browser, and seeing it behave differently? I assert that many, many problems are found this way. I further assert that ambiguities don't really matter to people if all browsers behave the same way - until you need to do something different (eg make something faster or add a new feature), at which time those ambiguities suddenly become critically important. Enumeration order of JS properties comes to mind here. The Web came to depend on creation order despite it not being specced. But what about indexes? What if you have some of both?