I must confess, this post makes me feel a bit like Edward Everett. In the future when I attempt this argument, I'll just quote you:

"A lot of us would prefer that every browser maker simply implements the standards, but a lot of innovation in web technologies (img tag, applets, xmlhttprequest) started life as browser-specific extensions. Frowning upon, or not allowing any differentiation would remove a lot of motivation for pouring resources into browser development."

Very well put.