Isn't the biggest problem that it gives one engine too much power over the web? Like IE in the first browser wars, it could start ignoring the web standards and implementing its own features, which are badly thought out and harmful for the web. For example, with a WebKit monopoly, suppose the WebKit authors introduce `window.badlyThoughtOutFeature()`. It's instantly a de-facto standard, and all future browser makers are obliged to support it for backwards compatibility. With multiple engines, nobody has too much power and can get away with things like that.
