I was thinking...
Mailto: links are handled by standalone email clients only.
Most users nowadays use webmail (like GMail).
Webmail can't handle mailto: links.
When a webmail user clicks a mailto: link, he gets either a confusing message about no email client installed, or the link opening in an email client he doesn't use.
Anyone can copy and paste an email address, considering the addresses don't always come in a mailto:.
Ergo, mailto: links are harmful and deprecated? Should I not use them when I need to display an email address?