I think you're mistaken in saying you can just share any file. You can't because there's no native metro file explorer, so I can't just navigate to any file I want to and pull up the Share menu.
On that note however, you could easily download a 3rd party file explorer that allows sharing of files.
This issue that they're talking about is a bug though. In browsers like Opera, it detects that the PDF application is the default application, but won't open any PDFs Metro-side. Same issue here, where Windows Send-To detects that the default MAIL CLIENT is the Metro Mail application, however the Send-To(which uses the mail protocol), does not work.
There's absolutely no reason it shouldn't work.
I agree completely. Windows 8 doesn't detect that Metro Mail is my default mail client? Ridiculous! This should be an easy fix for Microsoft.