I have read this thread and understand the original question (why is there no integration between legacy and Metro apps, when it comes to Mail) as well as the more advanced responses that followed suite.
I think the issue is that "Desktop" is a rather "divisive" and counter-intuitive app if one needs to install Outlook or Windows Mail in order to use "Send to - Mail Recipient" from File Explorer. Yes, the more technical ones amongst us understand the differences in OS, but seriously, Windows 8 is all about immersive App experience and users really need *not* have any technical pre-requisites to know how to bridge the 2 world like tinkling with "Default Programs" settings and that hassle. It really should just work.
The scenario is that one could be working in Paint and have saved something to a folder, hence is working in the Desktop/ File Explorer environment. Thus ideally, when one clicks "Send to- Mail recipient" for the selected files, say images, it should automatically invoke Mail (Metro) and bring the user there for further (text) inputs and ultimately, hit "send".
Microsoft, are you reading this?
zhudm