> We're all adults here and we know the consequences of reaching into implementation details.

I wish you were right but, IMHE, it requires a lot of communication once teams grow and many team member do not fully understand the consequences of what they do. It is nice to have something that helps when reviewing code.

> If you have members that users probably shouldn't touch, you prepend them with an underscore

Well, this is precisely what TFA does. It prepends the constructor with an underscore.