You'll get varying answers here.
Personally, I do add it. I started adding it to our spring 2011 chicks when 9 of them (all Iowa Blues) came down with pasty butt. I Googled what to do with it and found my information here... since I prefer to use natural methods over medicinal ones, I decided to give it a try. It worked wonders for them!! I forgot to add it to their water a couple of times and the pasty butt came back - added the ACV back to the water again and it stayed away.
Anyway, the many [alleged] additional reasons to add it is that it's supposed to help lower the pH balance of their digestive system, increases egg production, kills germs that cause respiratory issues, keeps the slime down in the waterers, keeps the harmful bacteria down in the waterers, etc, etc, etc....