It's definitely not something I decided on a whim. 99% of the IMing I do is via either FB, Skype, or WhatsApp, I don't use any data since free Wifi in my area is incredibly good, and I make about 1 phone call a month.
All in all I spend £5 a month maximum, which is fine by me. Not to mention the freedom of not having a contract phone.

I don't know if you're in the US, but don't you have to pay to receive texts and calls there on pay-as-you-go? Because we don't here, which would also make it significantly cheaper.