I don't see any issue at all if the bot is respecting the robots.txt file. Any malicious user will figure some other evil way anyways, be it Nutch or a network of intelligent lightbulbs.
User-agent: google
Allow: /
User-agent: *
Disallow: /
well, yes it's good behavior to actually respect it, but well I've seen such robots.txt already which makes it really painful to create a competing search engine.Edit: In my scanning, I have to confess that wikipedia's robot file is the best. Fairly heavily commented on why the rules are there. https://en.wikipedia.org/robots.txt