Generally it's a good thing for the web itself to have consistent standards and consistent adherence to them. On the face of it having lots of different browsers all having "their say" on how things display and then adding their own proprietary nonsense on top makes everything more complicated and is far from ideal.
The issue is not having "one rendering engine" but who has a say on what that rendering engine does. WebKit itself is open source. If the companies involved and the standards authorities developed clear, solid standards and implemented them properly it wouldn't be a bad thing. At the moment things like CSS recommendations take years.
Standards are good, but everyone developing only for webkit and only using webkit specific prefixes and features is a bad, bad road to go down. Yes, it wouldn't be as bad as the ie6 days, since webkit is open source at least, but its still the opposite of proper "web standards"