Actually, we had an option not to allow WhatsApp to use our contacts (we were given a heads up)and we used it.
Using WhatsApp is like sending text messages but it is more upgraded. It has become a valuable working tool.. at least it for me, it's an inseparable part of my work. You have the ability to send pictures and video clips easily, plenty of emoticons and you have an ability to form discussion groups for all kinds of purposes. This makes it easier to stay in touch with family members, neighbors, co- workers, etc.
It has some downsides of course, such as others can monitor whether you've read their messages and ignored them. Also, sometimes the frequency of the messages can get annoying..
But I don't know anyone who doesn't use it.