I run an events website and I would like to have a slideshow on a home page where paid event ads (marked appropriately) alternate with editor's selections. These would be very valuable inbound links, such as NYE events promo - one of the highest traffic times of the year.

We used to use OpenX and they allowed to use PHP code for ad inclusion that then generated simple HTML and images. I would love to have the same thing happen here. However! There is an opinion from another forum that I should not let the ads get indexed or the site will be penalized in search engines. I am confused - what would be the possible reason for this penalty? Is this true?