I have personally and purposefully caused a lot of confusion on some sites by using Cyrillic letters that look exactly like English letters to impersonate other people. This was mainly for fun and for harmless trolling, but it's very easy to see that this could be used on any site that uses Unicode for usernames, etc. Phishing is extremely easy with this and something needs to be done otherwise no one will trust the Internet ever again, especially if someone can just "steal" Whatsapp so easily.