I've said it before, and I'll say it again: blocking all but certain bots is the best way to block innovation. It's bad for you, it's bad for the ecosystem. Bad for the ecosystem because incumbents that want to respect robots.txt to propose new services will have an unfair disadvantage. It's bad for you because it'll just give more power to Google et al regarding your incoming traffic (and you'll have to follow their rules, like every SEO is doing right now).