2. If people don't code to the standards, it's much more difficult to make new and better browser engines.
1, expanded: All browsers have quirks, or bug, or specific ways of handling ambiguous situations. IE6 has more than most, but even modern browsers do have their edge cases and unsupported features and whatnot. If there's only one browser, people will code to the quirks, not the browser. Historically, the single largest category of implementation-dependent behavior that people rely on is how an implementation handles erroneous input. That's why malformed HTML is now standardized.
No, not you of course, but all those other web developers. Look, standards are nice, but to many developers (especially hobbyists), and to most suits, it's just a bunch of hot air. And the hard part is, they're right, at least in the short term. Coding to standards costs time, and coding to browsers produces a pixel-perfect functional site.
Multiple, disagreeing browsers are what make standards practical. Coding to standards is a best-practice, low-cost way of dealing with the real problem of divergent browser implementations. If that problem becomes theoretical, but the cost does not, then the sensible path is to code write pages that work in Webkit, standards be damned.
2, expanded: The more that existing code relies in the quirks of one implementation, the harder it is for a new implementation to arise. The surface area of features they have to match is expanded. Furthermore, these new "features" are more constraining to match because they are undesigned, not always consistent, and often closer to the metal.
Not only does this make it harder to write a new engine, it constrains the ways in which new engines can be better. The easiest example is how having to match another implementation's order-of-evaluation can limit the benefits of concurrency.
So the sum of this is that Webkit wins now, it will be incredibly difficult to get anything better than Webkit in the future (note that this applies equally to forks of Webkit--they're still limited in how much they can differ). Not that Webkit's bad or anything, but I think it's myopic to say that we won't ever want anything more.
Servo, of course, is right now the most concrete example of something that we might want in the future, with its promise of a fundamentally concurrent architecture. But even aside from that, the future has a way of surprising us, and I wouldn't like the be stuck with outdated tech in 2018 because webdevs in 2013 got a bee in their bonnet about webkit vs gecko.