It's many times more useful to use Skype on your mobile. If you're on 3G, it means you have no access to desktop Skype. And if it's more useful to you, they can charge for it.
> why should Skype arbitrarily charge us a second time?
1) because they can; 2) who's going to stop them (definitely not carries who gain on that agreement)
Now I'm just hoping someone comes out with a iChat/AIM/Jabber/GTalk-like P2P standard for voice (does this already exist?).