What are you explaining?
You say that IE6 has a standards mode that wasn't. So bad pages still rendered incorrectly*. IE7 was supposed to have a standards mode that isn't. So bad pages still rendered incorrectly*. But a different incorrectly. IE8 is supposed to have a standards mode that may be (this time). So bad pages render correctly.
(*incorrectly in that a bad page should give bad output, showing that you have a bad page.)
Now if IE6 required you to change your page from IE5 to meet the new quirks, IE7 required you to change your page from IE6 to meet the new quirks, why do you not change your page from IE7 to meet the new quirks (W3C correct rendering)?
You did it before.
And you didn't need a meta tag.
And early IE6 there weren't any competitors (hence it stagnated a long time: MS killed off the IE team and used them elsewhere because they had won the browser war against Netscape). So you HAD to render for quirks. Now, however, a large and growing section of the internet and intranet (because of cross-site-scripting security problems) have other rendering engines. Much bigger a market than IE7 has. MUCH more than IE8 has. So you have already a pressure to make your pages work on these browsers.
And if IE8 has good standards compliance, if it works on these other browsers, it will work under IE8 too. So even if you have to keep IE7 for those legacy systems running closed source applications that will not be supported (see how nice lock-in is?), there's no need to use this meta tag: you already have a page that recognises IE7 and makes a quirky page that "works" on it and if it notices Firefox or Safari or Opera, gives out a page that works with them.
Well, guess what? IE8 you've told us tells the site that it is IE8 and not IE7, so the code that detects IE7 in your site and gives it the broken pages and if not IE7 gives it the W3C pages can give IE8 the W3C pages.
No need for the tag.
By NOT using the tag, W3C standards are more widely used.
By NOT using the tag, if IE9 doesn't support IE7 (or 6, 5.5, ...), you have no change to make, because IE9 will be W3C compliant. Hey, lookie! No work treadmill!
By using the tag, W3C standards are less widely used.
By using the tag, if IE9 doesn't support IE7, you have to change again.
Or is that what you're looking for? A continuing reason for employment?
Now if you REALLY want IE7 pages to remain pristine and unchanged, how about demanding that IE8 isn't used and IE7 remain the browser for your company? After all, that way you don't even need to put the meta-tag in! That's a LOT less work!
Your parting shot isn't a reason for having the tag. If your page is broken, you're stuck with IE7 (or hoping like heck the IE8 version of IE7 doesn't change and screw up your specific page). If your page is broken as per the standards, change it. And users wouldn't notice a difference looking at your page with IE8, Firefox, Opera, Safari, Lynx, ... If you're really trying to appear as you're thinking of the "user experience", then enabling people to use the browser they WANT to use on your pages is what you should be looking for. And the tag doesn't let that happen. You still have to change your pages to be W3C compliant.