I would prefer not to send a message when declining an invitation to share details on Careers. The UI gives the impression that if I enter nothing into the Reply box, it will send a blank message to the prospective employer, which might make me look rude. At the same time, I'm not going to go to the trouble of writing a rejection message to every bad fit that messages me; this opens up a dialog when there is nothing to discuss, and I would rather spend my time following up on actual leads. But I'd also rather not leave it hanging because then I get e-mailed about it constantly. And I don't want to change my e-mail settings because then I might forget about a message I actually do want to reply to.
When you reply not interested, we allow you to send an empty message (that's the only time it is allowed). When you do not write anything, we still send an email, however it only states that you have rejected the request and includes none of the formatting we show when you have included a message with the response.
One of the things we strive to serve our employers with is a higher response rate, so we appreciate you not wanting to leave messages open. If you do not want to start a dialog, simply replying not interested with no message is the most appropriate response.