[ifette]

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 visual keyboards.

Let's say the Internet had been invented in Japan instead of the US. How would you feel if people told you that you had to write your name in katakana everywhere? As another commenter mentioned, internationalization is here to stay, and if we want to expand to the next few billion users it's even more important. FWIW internationalized usernames are already available on a number of non-email platforms (Weibo as a prime example). For email to remain competitive, it's important to keep up in the internationalization space.

[thaumasiotes]

Well, internationalized usernames are "available" on weibo in the sense that your displayed "name" can be anything you want. But you don't log in with your displayed name; it's an arbitrary bit of account data, and is changeable whenever you want. You log in with an email address, which is how the system identifies you.

(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))

[thaumasiotes]

I admit I don't understand the downvote. Email already has internationalization in the same sense as weibo does. You might receive email from me as 'From: Michael Watts <[e-mail]>'; the email address doesn't support arbitrary characters, but the name does (I've received email from '"=?gb18030?B?w8DIy7nY?=" <[e-mail]>', which worked out to a displayed name of 美人关). Similarly, if I wanted to display 美人关 as my handle on weibo, I could do that, but I wouldn't be able to use it to identify my account.