If you're charged for SMS messages rather than having a monthly allotment, services like iOS Messages/WhatsApp/BBM really help since they use your data allowance rather than being specific things the telco can bill you for.
For me, iOS Messages allows me to send text messages, pictures, videos etc to my friends in the USA (I;m in the UK) for free - all it is is data, which I have a ton of (infinite if I'm on wifi), whereas my carrier charges 10-25p for international SMS and picture messages.
RIM's BBM service is like that but previously was exclusively for blackberry devices (as iMessage is for iOS), with WhatsApp being cross platform. Going cross platform with BBM is RIM's attempt to keep blackberry marketshare, since people are really starting to cotton on to these sorts of messaging services as an easy way to avoid massive carrier charges for sending pictures and SMS messages. Their only real downside is that they tended to be OS-specific.