As the question states: Mailto is useful enough for people who use a desktop email client. That number is, I'm thinking, going down all the time outside of corporate intranets. Thus making mailto links an annoyance for these people, as they have to close whatever the default mail client is on their system and copy-paste the link.
Now, I know that there's plugins to bridge the issue, but let's face it - most people probably don't use them. Is using mailto rude or unthinking these days? I say yes, what say ye?