I was mostly referring to your comment about sites that sell something as you seemed to differentiate between directories and ecommerce sites. If a visitor is going to buy from you online they'll buy your product regardless if there's a similar item elsewhere. Perceived value and an USP will get you the sale. But considering the low number of visitors that actually convert, it's easy to understand why retailers are attempting to somehow monetize their traffic and AdSense is one option. One example - I had a client that offered unique items online, the average conversion rate was approaching 5% which I consider very good. 95% of her traffic was just dead traffic. She didn't want to add AdSense for the same reasons you mentioned. I convinced her to do so as a trial. Her conversion rate is still the same, her sales volume has increased (due to overall trafiic increase) and she's enjoying a side income that has averaged $500/mo. for almost 4 years.
That's what we need. Directory owners with principle. PS. People should really try to work on their site to make it look good and informational.