I'm sure I'm not the first to think of this and I think I even fell for it on someone else's home node long ago.

And this was fixed a while ago, because if you can send a message (to a user or the chatterbox) just by a simple GET request this is open to CSRF. Actually before this was fixed you were able to put an image tag in your home node and its source was a link to sending a message, so it would have been automatically called when you visited the homenode.
So it's good that this doesn't work anymore (although it's still not fully CSRF protected).

Why the prefilled form requires to edit the fields I don't know; it might also be a kind of protection but makes the msg link kind of useless.
Update: The prefilled form requires to edit the text field only (not the recipient, like I first thought when looking at the HTML source), and this is probably meant as a protection, so that people really look at the prefilled text before sending it.