If you're worried about Google being a bad web citizen, they've already said that they only use prerendering for the tom search result, and only if they think i

If you're worried about Google being a bad web citizen, they've already said that they only use prerendering for the tom search result, and only if they think it has a very high probability of being clicked. (Based on my limited testing, this only ever seems to happen with navigational searches.) So you'd be spending that bandwidth anyway, and your users are getting a better experience out of it. If you're worried about the technology itself being abused to DDOS people, I don't see how it enables any attack methods that didn't already exist.