It also makes it hard for users to log in using a dumber input device than usual. Then there is unicode normalization. OSX might decide to encode "e-with-an-ac

It also makes it hard for users to log in using a dumber input device than usual. Then there is unicode normalization. OSX might decide to encode "e-with-an-accent" as two code points, while Windows will combine them into a single code point. Users will not be impressed if they can't log in because their OS doesn't use the right encoding and the web site forgot to normalize.