It's $49.99 a year, though. As a fairly heavy flickr user I could easily see spending $4.16/month to remove ads.
Pretty sure it is not a business feature to show a clean page but a viewer option for distraction free viewing.
And it seems that after proving people will happily pay for premium features, they've now sent an email to all their customers which essentially says unless you have over 1TB of photos with us (which is pretty-much no-one), you may as well cancel and use the free account instead.
It seems like a very bizarre structure to me and I can't see people subscribing to it which is a shame as IMHO they've just vastly improved what was already by far the best product in its market.
I Hope I'm wrong.