Malte-
I agree their are a bunch of reasons you would want to turn the feature off. And, for the most part, I would expect that in most of the situations you cite, the feature would be turned off.
But for sheer viral marketing potential, it would be a very good feature. Now, of course, I have to put my money where my mouth is and put out a proposal (I'm not a developer) and I'm not quite _that_ in love with the idea yet. ;)