Today there's an email to the www-svg list which illustrates perfectly the value of browser engine diversity. This kind of thing happens all the time.
To be clear, browser engine independence pays off here because it tells the Web developer that there is a real choice of behaviors. The developer can say "oh, that behavior is definitely better" instead of just "oh, how strange". It also gives us implementors confidence we can change the behavior since Web content (if it's been tested in multiple browsers) can't be depending on one particular behavior. It also tells us that the Web developer's preferred behavior is actually implementable.