I’ve been thinking about the same question. I think the only real “issue” with doing it this way (which is indeed very awesome!) is the code bloat it produces instead of a condensed block down the bottom. As you have stated you would need a significant amount of media queries/large web site before performance would be a worry.
Dropping queries in exactly where you need them is fun, convenient and much easier (IMO) which definitely outweighs the downside of code bloat, when you add gzip into the mix you’re basically covering yourself in that regard.