Note: this question and image have been copied with permission from Bothsider (http://bothsider.com/i/366/)
I'd agree with 17's answer here, no. But maybe to elaborate a little more I would step back and ask why this option exists in the first place:
If the intent is to inform you of the content that is being sent to your recipient
That should be treated as a special case and not the default. By default the user should be able to trust the system to send something meaningful (considerate software). With that said, if that's the issue there's probably a more meaningful way to get there than just this checkbox. This dialog could easily show exactly the content that will be sent and allow the user to modify it without taking up much more space or cognitive load. That negates the need for managing another email altogether.
If the intent is to allow sharing with yourself
The active user is already reading the article in question, so the default behavior of any kind of "share" activity would be to share with someone else. Sharing with yourself is possible here, but should be treated as a special case. Question: How do I fill out this form if I only want to send it to myself? The label on the checkbox becomes unclear at that point. So in this case as well, the checkbox should be disabled by default.
My first instinct would be to say no. The sender has already read the article. If they are interested in referring to it later, it is likely that they would bookmark the link or get it from their browser history.
I say keep it checked by default.
Here's why: The expected behavior of email clients is that a copy of an email sent goes in your sent folder as a record. Since the form is essentailly a pared down email client with only one feature: send. Since it can't manage any folders, the "send copy to self" is a clever way to emulate the sent items feature for the sender. The emergence of the send copy to self feature on share forms indicates the feature is valuable.
Studies indicate people tend to ignore defaults. (I cited Dan Ariely's blog which is a good launching point for the research behind this. )
So if it's checked by default, users don't have to understand or decide what to do. Unless they actively DON'T want a copy. But, they reap the benefit of the "sent item" like feature of the form with out any bother.
I'd say yes.
Advantages: the user can see exactly what was sent to their friend, and the user has documentation of sending it.
Disadvantages: adds to the user's mailbox clutter by default.
Email can be deleted, so minimal user effort can remove the disadvantage if it's left ticked by accident. If it was the other way around, and was left unticked by accident, you can't gain documentation or see what was sent. Leaving it ticked by default reduces the risk of an accidental action that can't be undone.