One More way to squeze in One More ad impression
Many ad-driven sites have photo galleries. They're great because they garner easy impressions because the uses must advance (click) through to each next photo, turning one visit into 10 (or more) impressions.
I was logged into my "my.yahoo.com" home-page-portal-thing (in which I encountered another misfortune/bug I can blog about later) where I followed a link to a "Bit Kit" photo gallery showing off various cases for carrying and storing drill bits.
The slide show navigation did a wonderful job of setting the expectation that there would be 10 related photos in this gallery. I clicked and clicked my way through the gallery, each click confirmed the expectation... "Slide 1 of 10", -CLICK- , "Slide 2 of 10", ect...
For me, the final photo in a photo gallery is particularly fun to anticipate as sometimes It's the climax of the gallery and I'm often disappointed when it isn't. The concluding photo in this photo gallery was an anti-climax if anything. It was a freaking Advertisement.
IMHO, a site's objective from a usability perspective is to give the user a predictable experience. You build up expectations through consistent design and functionality and then with every new page and every user interaction the site confirms the expectation, the product being a predictable site.
It is dishonest to build that expectation that with each next click a new photo in the gallery will appear and then, on the final photo, rub the user's nose in the fact that they (the user) just put a few advertiser dollars in the site owner's pockets.