This is a very interesting article: http://blog.cloudfour.com/the-ems-have-it-proportional-media-queries-ftw/
This feels like one of those things that could become a best practice. And while I'd love it to be in the initial Zen 7.x-5.0, I fear there may be some subtle complications when browsers have different default font sizes. (The comments of the article hint at the complications.)
I think we'd need to test this pretty thoroughly before committing.
On the other hand, we're only setting the breakpoints for the default "responsive-sidebars" layout. Themers are going to need to discover the intrinsic breakpoints of their content and design, so they'll need to modify the breakpoints.