I wonder if there is a psychological effect with that along the lines of: what we perceive as being cheap/free can more easily impress us?
So, by watching a movie that you "would never have paid for" i.e. wouldnt have lived up to the value of the ticket, but they "end up being really good" because you have lower exceptions due to the reduced entry cost.
I dunno, but it is interesting
Can see where you're coming from and on the flipside i've ended up seeing some absolute rubbish.
Best example I can give of a movie I wasn't bothered about that ended up being amazing is Sicario, it's my favourite movie of 2015 and in my top 10 movies of all time.