I am a Dane. We use the latin alphabet plus æ, ø and å. Some names, like Søren or Åse, can't be written in pure ascii, but the people who have these names tend to just have addresses like soren@whatever.dk or soeren@whatever.dk. It seems preposterous for me to risk breaking the web over something so relatively trivial. Heck my name is ascii compatible but it isn't available.
A lot of cases of "accented characters" are much simpler, compared to e.g. Chinese or Japanese, where the mapping is not 1-1 (a given kanji could have multiple readings, or for Chinese, pinyin is not 1-1). There's also a number of people who don't know the ASCII mapping for their language. Chinese can be written with bopomofo or 5-stroke input, for example. There are programs for input of indic languages that use vis…
(checking now just to make sure, I see that weibo allows three options for logging in: an email address (not internationalized), an account number (not internationalized), and a phone number (not internationalized))