> So like it or not I am inclined to only allow Google and Bing in the robots.txt because Google is the only site that sends a significant amount of traffic and because Bing sends some, and Google needs some competition.

This sounds like you're onto a reasonable “legit” factor: does the crawler honor robots.txt? Baidu would be legit because they don't lie about their identity and if you put a rule in your robots.txt file they'll honor it.