Indeed.. the usefulness of Whatsapp is the fact that one can talk to friends on most devices ios/Android/BB/Windows/Nokia/Symbian. Anything else ,security, annoyances etc.. are easily ignored by non-techies.
There's another crucial thing that is missing from WhatsApp that is owning/backing up your message and of course it can rule IM space it jumps into web chat. But then they will have to implement an entire identity management system because in reality they do not have one.
I find WhatsApp good. Just talk to your friends. That's it. No celebrity feeds (sth called Friend+ or so) like that in KakaoTalk etc.