Apparently the rationale includes...
“Producers feel, rightly or wrongly, that that interruption, if you will, is going to lose viewers... one of the things that has squeezed themes out is this relentless kind of move toward tightening everything, making it go right from joke to joke, from action to action, from shootout to shootout, so that you won’t press the dreaded remote control.”
Thanks to the elimination of commercials between the end of one show and the beginning of another, shows overlap before fickle viewers have a chance to channel-surf to Another Network. More commercials air within a show, making episodes shorter. Main titles and well-rounded theme songs and scores? Sorry, no time, no money.
… “The networks sort of assume we watch the show, so we don’t need to have the premise explained to us each week ... In the era of the DVR, half the people watching the show are just fast-forwarding that anyway,”http://today.msnbc.msn.com/id/15320031
(I still get a chill thinking about the I Spy, or Mission Impossible theme songs.)