This debate boils down to two simple questions: What is our job as developers, and what is the purpose of the sites we build.
Our job as web developers is to create websites that are accessible to the target audience and convey the message to those audiences in an easily digestible way. That means we have to make solutions that work for the end user. The purpose of the sites we build is to convey information.
We have no (nor should we have) control over what tools, devices, or browsers people use to access our websites. Our job is to make sure that the information gets where it's supposed to go. Because WordPress is a CMS used to build websites for more than just Mac users in North America and Europe, we have to make solutions that work for the end users. That means graceful degradation within reason. IE8 is well within reason.
For reference, the main reason Twenty Twelve's menu gets all messed up in IE8 is because the default menu is the mobile one while the 'regular' menu is in the media queries. A simple swap of the style code fixes this issue. The way it's laid out now assumes only HTML5 - ready devices will access the site. This is an unnecessary restriction and as far as I'm concerned it's non-standard. The best practice is to make the 'regular' menu the standard style set and place the mobile small-screen menu in the media queries.
@shapeshifter With IE8 we're not talking about graceful degradation going on forever. As the world upgrades to Windows 7 and 8 this problem will disappear, and disappear fast. However, we have to address the current situation, and the current situation demands IE8 support. Google is dropping support for IE8, but they are not making their experience unusable for IE8 users. They just won't get the latest features. This on the other hand is tantamount to punishing people for not buying new gear.