I see one potential downside of this, as a Netflix subscriber I do find the frequent shuffle and changes of cover art confusing sometimes as it makes you feel there is a new film coming out but in fact you've already watched it.
And to be perfectly honest, sometimes those changing stills have actually made me watch a movie that I had previously glossed over because the image wasn't as interesting. So from personal experience I can say their primary intent was reached w/ the periodic cycling of stills.