The problem goes much deeper than just HTML fields - sometimes the punycode will be converted to Unicode and then get rejected further down the chain. For example, Google accepts punycode domains for custom email domains but will convert it to a Unicode domain name when receiving it. It handles most IDN domains fine, but it fails at domains with Emoji within it (my domain which fails is http://xn--p38h.ws).
The real problem here is all the systems we use are more complex than people realize. Things like names (http://www.kalzumeus.com/2010/06/17/falsehoods-programmers-b...) & time (http://infiniteundo.com/post/25326999628/falsehoods-programm...) just cause issues that most developers will end up just ignoring, because in the end its just edge cases that will happen maybe once or twice, ever. Why put in the effort?