I actually did this exact robots.txt for my site this summer. The net effect? A massive loss of secondary market traffic - enough that, after a month, we probably lost 10-15% in revenue. It surprised us. We rolled the old robots.txt back into place and bam - life went back to normal. You can question my pithy 300-character explanation all you want but I'll leave you with this: we were 100% certain that the robots.txt change was the difference.

In the end, we switched to network blocks for the common bots/spiders. Much better.