Relying on User-Agent represents the very antithesis of [[Postel's Law]], a rock-solid principle o which the Internet (used to be) based.
RFC2616: 14.43 User-Agent The User-Agent request-header field... is for... automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations.
Yes, that's precisely the violation of Postel's Law I was thinking of.