I think they should have both shorter, and longer frame matches. The Worlds is presitigious because it is longer, and different to other tournaments, and these matches tend to be more tense, thus more exciting. On the flip side, it can be annoying to watch an opening session, and the concluding session is when you are at work the next day. It's good to be able to get home in the evening and watch a whole match.
On saying that, I think Barry Hearn is getting a bit ridiculous with his 'shorter frames = better snooker' theory.... Best of nines are the shortest it should be. Best of 5 have no place in ranking tournaments. And the final of a ranking event should be far longer then best of 9. He'll have a Pot Black ranking event next.