I get your point. However you just don't send to anyone. For instance have you ever sent me an SMS? No. Why? Because you don't know me. That is why I said to have the best two BBM apps and have your friends do the same. You can't send a message on BB with BBM to someone you don't know, you need their PIN. I am not sure about PING! but WhatApp allows you to send messages internationaly for free, can you do that with a IM app? I don't know the answer to that, I'm just asking.
Yes, you can send messages internationally with any IM app because all IM apps send messages through data.
And, if more apps like this start appearing, each with different features and prices, it might be hard to choose "the two best ones". Some of my friends might be willing to pay for apps, others might not. Some might prefer the app with a best UI, others might prefer the apps with the best features. Also, if I have two different apps, what if I'm "messaging" with two friends, each one on their own app? It would be downright confusing and annoying.
Again, I'm not criticizing the app, I love Ping!, and I'm recommending it to everyone. But if a lot of similar apps start appearing and people choose different ones it defeats the purpose of it. The purpose of something like this is that you can communicate for free with anyone who has an iPhone. That's what BBM is for the Blackberry.
My point is that I hope that there is one app, either released by apple or some developer, it could even be this one, that is the standard "BBM for iPhone" app. Otherwise I'll just stick to regular IM.