[1] http://blog.whatsapp.com/index.php/2012/06/why-we-dont-sell-...
Secondly, that it was created to exploit people's laziness - i.e. it skips registration step (one time thing) presenting it as a "convenience feature", but it does it at the cost of sacrificing security (i.e. it offers no way to choose user ID and password, instead of that it uses hardcoded information derived from the phone hardware ID), obviously not letting the user know about this downside. So it's a wicked trick which is used to attract unaware users.
I use Whatsapp once per month and I'm not even sure what it does or how it does it. I just know I can send group messages to everyone in my contact list who have it and it WORKS.
When you sign up for e-mail, do you think it's normal to skip the registration step and to give you some random ID and password? Why is IM any different? Also, do you think it's normal for your e-mail ID to be tied to one device, or you'd expect to be able to access and read / send it from multiple devices / computers? Why should IM be any different? That's what XMPP is about. Whatsapp tries to create a crippled service instead, but somehow manages to wrap it as a "convenience".