> Why is that a good thing?

Because it is useful to have a communication mechanism which is universal. It is a standard and not owned or controlled (censored) by anyone. Sure there a dozens, possibly hundreds, of fragmented chat protocols that can't talk to anything other than their own proprietary clients. But only email is universally available.

> only friends I've chosen to let in

You could configure your email client the same way and then you have the same behavior.