How would that make more sense? That would cause everyone who wrote a website between 2001 and 2007 for ie6 or ie7 to have to go back and add <insert special tag here> to every one of their web pages just so those pages would use the rendering engine of the older ie browser, they were originally written for, in ie8.

On the other hand, if you are building a new website, then you should have no problem adding in one extra tag because you are going through the trouble of making sure the pages render correctly in all browsers anyway.

Meta tags have always struck me as sloppy anyway. And it's not like they're asking you to add something new to the top of the page that replaces the doctype. Adding something as trivial as an extra meta tag to your "new website" should be a non-issue. But backwards compatibility being preserved for a newer version of a browser viewing older web pages without forcing a developer to update all those old web pages makes the most sense to me.