Yeah, I went to their homepage to see what the app is and this is their first paragraph,
WhatsApp Messenger is a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for iPhone, BlackBerry, Android, Windows Phone and Nokia and yes, those phones can all message each other! Because WhatsApp Messenger uses the same internet data plan that you use for email and web browsing, there is no cost to message and stay in touch with your friends.
Maybe this is still relevant internationally but here in the US we are no longer charged for SMS and are actually charged for our data. This app makes no sense to use for anyone I know. I guess that makes their decision to drop older OS support even more asinine because the people on those older phones are the ones most likely to still being charged per SMS maybe?
I certainly don't care.