If a recruit says "I'm committed but still willing to listen" then you find situations like what you mentioned with Latu and Kaho. This is largely a result of players "committing" early but not being truly committed to the school.
However, if a recruit is 100% committed to a school and has very little or no interest in switching to somewhere else then I'm sure it does discourage other teams from actively recruiting him and thus giving him fewer offers. I believe Mangum falls into this latter category rather than the former one.