I have write a lot of open source software. I am OSS supported as the next guy, and for OSS standards are very important.
But standards are not more important than progress. With the current system, browser creators can invent any fun stuff and add it on the next update this week. And it don't break anything. And theres nothing bad in that.
About webmaster using it, what is wrong is the level. Wen a JQuery extension or a CSS library (think... reset.css) use a extension, is Ok, because it abstract a problem, so the normal webmaster don't have to know or use all these -moz- -webkit- etc things. These things HELPS, helps so much that we can fix broken things on the web using then, like... making all browsers act the same way. And to do that, sometimes we need a library author, or a JQuery plugin author to put his hands on these extensions. Another similar thing is how you don't have localStorage of HTML5 support everywhere, but you can use it everywhere trought a library that uses something else in browsers that don't support it ( IE has something similar to localStorage from IE5, that is superugly but can be forced to provide the feature).
These extensions are GOOD, but should be reserved to be used by the library authors, not the general webmaster public.
This proposition is WRONG because sometimes you may want to support "opacity", but your programmers can't support exactly as speced, is better to have a --ie-opacity than have a opacity that work different than the standard. The browser extensions allow browsers to support things in a dirty/not complete mode, withouth breaking standards. This proposition is wrong.