I disagree. It depends on the directory. If you run a good quality directory, AdSense is fine. If it's something spammy, Google might not like it too much.
I have 3 directories, and never added it because I assummed it wasnt but sometimes see directories that have it
I have couple of directories and I've never had any problems. Of course directories must be good quality. in Poland there's a script called "qlweb" - thousends of directories based on qlweb are in the web and I see that lots of them display ads.
I tried running adsense on my directory in the past and didn't get much out of it. Adsense may work better if you had a blog attached to your directory. Then update the blog weekly or daily with fresh content. Just a thought. When i visit directories, i am there for one reason only and not there to view a ton of advert's. (Just my own opinion)