Earlier quoted context omitted.
I think it matters, quite a bit actually. My expectation as a user is that the URL I see in the bar is the URL of the site I'm visiting. If it's not accurate, then why show it at all?
Because the second level domain is far more important than the subdomain. The second level. www.example.com and example.com usually have the same contents, but are always controlled by the same group.
It seems like the confusion this change causes negates any benefit it could have.