The less teams you have in your final series, the greater the number of pointless games towards the end of the season and the bigger the impact on crowds.
This is not as clear as most think.
Firstly, under a top 8, most of the top 4 or 5 know they are going to make the top 8 from a long way out, even from the start to be honest. And they know they are going to do so whether they win or not. This means that it really doesnt matter whether they win or not. And while it is true that it is better to get a higher position, it is not really until the last couple of rounds that this is thought about.
Second, The top 8 becomes separated from the rest just as regularly as the top 5 does, meaning that the number of end of season meaningless games at the end of the season is about the same in this regard, especially since a top 3 in the top 5 system is far better than a top 4 or whatever sides are after in the top 8 system.
Third, One problem with a top 8 is that you really have to play badly to miss out. I can t think of any teams who play well and have a good season and miss out. In fact, by the time the finals come around, the teams who are on the fringe of the 8 really are so disillusioned with their side, that many of them dont care by the first round of finals. This is changing over time slightly, but it is more a case of people hoping for a lucky run than getting excited over the teams form. Under the top 8, you really have 1 winner of the comp, A runner up the rest are losers and rarely get any satisfaction from the season. Under the Top 5, there is one winner, and in almost all occassions the next 5 despite being dissappointed all are satisfied with having made the finals, as it is a huge achievement. In fact, often sides who just miss out on the finals feel they have had a very good season.
Fourth, The game is robbed of block busters. 1 v 2,3,4 games mean very little nowadays. With a top 5, these midseason games are absolutely massive affairs with huge meaning and huge consequences for the losers. Now they are decent games but mean nothing at all until the finals series. The top 5 really does change the way the whole season is looked at. Yes, it keeps the season alive for sides like the Titans (this year), for a little longer, but so what, the Titans fans had given up hope, long before their mathematical chances eliminated them. In fact, most sides running below 10th have given up a while ago. In a top 5 system fans rarely gave up, while they had a mathematical chance.
fifth, Let us not forget that the top 5 often has 6 teams in it. The midweek playoff was an integral part.
6th, A top 5 means that there is only 2 games each week. This means that all teams playing get a grand final (almost) style build up to there games. Everyone concentrates on every game and every game is huge and close, as opposed to now where round one of the finals feels just like an ordinary round, because there are so many games.
I really think that those who like the top 8 over 5 just havent experienced both. Very few people who experienced both actually prefer the top 8, and of those who do, 90% support sides who have really struggled for most of the time.