Please correct me if I'm wrong, but my impression is that it's not just that you don't see ads; it's also that others don't see ads on your photos. The latter can be an important feature for those who use Flickr to create professional portfolios.
The original pro account removed ads while browsing the site, Not from your pages as viewed by others. Plus a lot of the time your Photos are submitted to groups, would adds be removed every time one of your pictures turned up on the page?
Pretty sure it is not a business feature to show a clean page but a viewer option for distraction free viewing.
Anecdotally, I think it is the case that if you're a Pro user, and someone else is viewing your photostream, then they won't see ads next to your photo. This probably doesn't apply to your photos in other groups, etc. So we both may be right.
