This proposal is completely wrong-headed. Wikimedia's goal is to provide free information, to everyone. If it can do this helpfully and at reasonable cost by providing free image hotlinking of images it happens to have anyway, it should do so. As for arguments about costing Wikimedia money, please see http://en.wikipedia.org/wiki/Wikipedia:Don%27t_worry_about_performance. If the sysadmins think it's a problem they'll handle it, and it's not.
If some of the images people are hot-linking are non-free, it's up to the ones hotlinking them to determine whether their particular use meets the requirements of fair use -- Wikimedia need not police third-party use of its resources unless there's a complaint. Others have a right to use copyrighted works under fair use, and Wikimedia should not stand in their way. And if people are uploading images that are useless to us so they can hotlink them (which I haven't heard is a big problem), just delete the images.
There's no need for selfishness here. Wikimedia's goal is advanced about as much if third-party sites can use its information directly as if they have to direct people to Wikimedia's sites to get it. The point is to disseminate the information, not to disseminate it with a Wikimedia-owned logo in the top left corner of the page.
Essentially the same arguments apply to watermarking and other overly protective measures. The status quo is perfectly fine with respect to third-party use of Wikimedia images. I've hotlinked Wikimedia images myself on more than one occasion.