Agreed! The constant focus on browser security means that this kind of thing is avoided out of hand even when security is not an issue (for internal deployments for example).
I am really arguing the opposite point. There are lots of situations, particularly in business where you want to be able to interact with the file system and OS. Currently your options are very limited if you want to use a browser as a UI. How do you add a "send as attachment using outlook" button in a web page? Native client does not help with this due to the sandboxing.
