The shim won't fix the quirks mode, but at least it will make sure all your CSS works.
Here is a testpage. Load in IE 9 or 10 to see the issue.
Why would you have a page in quirks mode? Well me and you wouldn't. But you can't control every pageon the internet.
At Wufoo, we allow forms to be embedded on third party sites in iframes as well as use HTML5. We recently just had to make the changeto load the shim on all versions of IE because of this very issue.
This is relevant to literally any page that could be embedded via iframe. Off the topof my head: a calendar like how you can embed Google Calendars. Campfire chats. Embeddable widgets in general. Flickr slideshows. Other forms like Dispatchor polls like Polldaddy. Twitter and Facebook buttons come over in iframes. Tip of the iceberg.
For the IE team
Can this please be fixed? It's likethis in IE 10 platform preview too. It's unsettling to know that people can iframe your content and have it look totally bungled through no fault of your own.