Webmaster's shoot themselves in the foot a lot. "My site isn't showing up in google" is a frequent complaint on webmaster help forums, and typically the problem is robots.txt or meta noindex. From that standpoint, since most sites do want to be indexed, it makes sense to follow the standard as strictly as possible. It should be hard to remove your site by accident, which case insensitivity would make somewhat easier. Google states explicitly in their robots.txt policies that it is handled in a case-sensitive way.