Advertising is a business of story-telling in as few words (I must add seconds also) as possible. If it takes longer and longer (both words and time) for an agency to tell a story, they definitely are not masters of their art. In current scenario, I am slowly coming to a conclusion that if an agency can't tell a story in 20 seconds flat, they are either too lazy or too dumb!
Average attention span of a consumer is declining at the rate of speed of light. With a stressful life at every step, who has the time to concentrate for long on advertisement of a biscuit, or hair oil, or soap? If something doesn't grasp her attention in first 5 seconds she changes channel. Similarly, if something doesn't tell its story quickly (20 seconds) she changes channel. Patience to watch irritating and long commercials is not the cup of tea of current consumer. To grab her attention, you need to be a master in the art of quickie. Else she will become trigger happy and kill all your investments in production and airtime.
An off-shoot of this rapidly declining attention span is the growing importance of frequency. It is not that reach is not important; but with a disinterested consumer, the only way to get some attention from her is to bombard your advertisements on her as many times as you can without irritating her too much. In the current scenario, I feel that one can trade-off reach with frequency to some extent but trading off frequency with reach would be catastrophic. And the best bet to increase frequency is to have short and sweet creatives to get the most mileage from the spending on commercial time. By reducing length of advertisement from 80 seconds to 20 seconds, one can increase the frequency by 4 times (assuming everything else as constant)!