I don't think men have an erection throughout the entire duration of watching videos. I think everyone has specific likes and dislikes and I don't believe it'd be possible for any video to fulfill each individual's tastes throughout, however I do think that most videos are arousing enough that the man would not go soft through the video, but rather hover between very hard, and semi-hard.
 
In regards to why there is so much spitting, and licking - lubrication feels good for most people, and rather than reaching over for a bottle of lube all the time, a little spit does very nicely. Also, sex for many people is something where they can really let loose and become almost animalistic. Spitting releases some carnal imagery and feelings within us, and allows us to almost become something else - something outside the norms of daily life, and generally accepted social behavior Remember sex is supposed to be fun, raunchy, and dirty. Let it take you places you can't normally go!
Simon, 20 Something...

The spitting can both be used as foreplay and lubrication and yes the men usually have an erection the entire time.
Scott, 30 Something...

Having directed hundreds of adult films, I can honestly say that the men DO NOT have erections all the time.  They time themselves, fluff themselves, and sometimes take Viagra to get it up at the right moment.  They are not Supermen!  They are professionals and have little bearing on how "real men" are in every day life.  The spitting thing is a current fad.  I personally hate it, but it seems to be popular right now.  Why?  I have no idea.......
Jack, 40 Something...