There has been a debate about browser version targeting for some time. Browser version targeting is a scheme devised by Microsoft whereby a website declares which browser it is designed for, and it's then the browser's job to display the page in the appropriate way. If Microsoft proposing a new standard doesn't make you at least mildly suspicious, you must not have handled Microsoft and non-MS interoperability.
You might be asking, "Since DOCTYPE declares how the page should be viewed, isn't this redundant?". If so you are right, this is redundant. The trouble is that browsers don't always support standards, one browser in particular comes to mind, of course. To be fair other browsers haven't always been perfect either, but by and large, the worst offender has been IE. I have gone through much pain getting web pages to display properly in IE. I grudgingly hold Microsoft accountable for many wasted man-hours, and that's just for me. One of my last design stages is IE testing, and fixing things which work in every modern browser except IE. Consider how many wasted man-hours Microsoft is responsible for world-wide, just for defying web standards.
I suspect what Microsoft is proposing isn't a way to version target web browsers. I suspect they are proposing a way to version target IE. Microsoft never cared about standards compliance until it was forced to because their customers were using an inferior product and people started defecting to using Firefox, if not jumping MS-ship altogether. Do you actually expect that a webpage targeted for Safari 4 or Mozilla 3 will be displayed specially in IE9? This is too much to expect from the software goliath. This proposal by Microsoft is just another way for Microsoft to escape standards. This tag anticipates that browsers will not obey web standards and asks "Which web-standards broken browser are you using today?". It will be up to all other browser developers to embed IE-quirks into their coded, and that's something I'm not comfortable asking the Mozilla Foundation to do. Further, there is no guarantee that future versions of IE will actually be W3C standards compliant.
If MS wants to use a meta tag so that IE developers who have myopically chosen to program proprietary webpages can keep functionality across upgrades - that is their business, and fine with me. However I recommend that standards-compliant web developers ignore this meta tag. If your web page validates, and IE-users have trouble viewing your page, I recommend you leave it broken. Advise your users, as I do, to switch to a standards-compliant browser.