I can't imagine any scenario of a widely used email app opening a hyperlink that automatically composes and attaches local files being acceptable, even if they have to click send.
Our users want to be able to be able to open a declaration and email the documents, either internally or to their customers, along with some message.
We don't really want to implement a full email client in our program nor having to deal with their SMTP server setup, including proxies, firewall and so on. So we basically do exactly this, open a new mail in their client with some template text and with one or more attachments.
We use MAPI though and not "mailto:", but use-case is there.
(Addendum: See spacebar heating macro XKCD.)