It's a huge issue on mobile. Multiple rendering engines requiring the world to write software to a spec is what prevents people from writing code dependent on a particular browser or, in this case, a rendering engine. You see it all the time in the mobile web: plenty of sites just plain don't work when using other rendering engines.
And the fact that web developers then actually can't develop just for one rendering engine and claim that "it's enough" is a very good thing for me. I refuse to use Chrome and I'm extremely hit by all the sites developed "for Chrome only" on the desktop. The web shouldn't be "what's on the developer's machine and not more."
