Why use Facetime when... You can use Skype, and not limit yourself to an Apple product.
Eg. I can Skype (for free) between iPhone, Mac, PC, Android, Linux, Wind0z, and probably even Windows Phone.
morboComplete Your Transaction
The same reason people use Internet Explorer: it's the default application. Apple includes it with their products, so people don't have to download any new software.
fiberguyMy views are my own.Premium
.. like people with iPhones don't download any new software.
The main reason, in my opinion, is that it's integrated in the phone and just works. I have long avoided skype because it had always set up, on the web, like trying to perform brain surgery.