Yes, they are. The $/MB is insane for texting plans.
I also don't know why you think there is going to be an explosion of data usage. No music streaming, and I'm sure most of the syncing will be Wifi only (home network near computer) with the explicit option to choose the data delivery method for such operations.
That being said, it's not as if this is anything new. Carriers have supported BB's implementation of the same type of feature for years without fuss.
The thing that is new is that you don't have to separately configure the other app, you don't have to pass around another identifying token for the other app, and you don't have to go to a different app to talk to BBM users relative to non-BBM users.
The big innovation (if you want to call it that) is that Apple is auto-routing the concept of a "text message" to the cheapest available network. BBM didn't do that, which is largely why BBM was not shunned by network providers.