The idea is that we don't have to test in IE8 at all for now. I tested the main templates in beta 2 in standards mode and only two things broke (a negative margin on the nav and a min-height). Is it worth locking ourselves into IE7 mode just for that?
I'd fix the two bugs. IE8 standards mode is the first IE in years that does what the spec says most of the time. Pinning everything to IE7 means you keep every IE7 bug you already work around, forever, or until someone remembers to take the tag out.
Counterpoint: it's a beta. Microsoft has changed rendering between betas before. For a big site with lots of templates I can see why a lead would want the safety net until the final release is out.
Keeps Clearfix tidy. Ask a moderator if a post goes missing.
We did both: the meta tag goes out with the next release, and there's a ticket to remove it and test properly once IE8 final is out. Writing the ticket is the important part. Otherwise it stays there for five years.