I'm not at all against it. It gives teams like USA and Canada a better chance at getting a victory come RWC - so some of those third tier teams have games they can be confident of being competitive in. It also allows teams which are growing but haven't yet fully pushed into that top 20 - teams like Spain as an example - get a chance to compete and grow interest in those countries. I'm personably in favour of expansion. Yes there may be an additional blowout per pool - but there will also likely be an additional relatively competitive game per pool for the teams currently seeded bottom.
Based on the current World Rankings here is what a 6 pool 24 team tournament would look like:
Pool A: 1. New Zealand, 12. Fiji, 13. Tonga, 24. South Korea
Pool B: 2. South Africa, 11. Japan, 14. Italy, 23. Portugal
Pool C: 3. Ireland, 10. Scotland, 15. Georgia, 22. Namibia
Pool D: 4. England, 9. Samoa, 16. United States, 21. Spain
Pool E: 5. Wales, 8. Argentina, 17. Romania, 20. Uruguay
Pool F: 6. Australia, 7. France, 18. Canada, 19. Russia
Canada have France, Ireland, Italy and Romania in 2015 while USA have South Africa, Scotland, Samoa and Japan. I don't think under the 24 team tournament that either team really has a better format. I'm also not sure how many of those games between the bottom two in each team would be very close.
I think that the current 20 team format enables a good range of abilities at the World Cup. One of my favourite things about the World Cup in 2011 were the competitive pool games. Ireland vs Australia and South Africa vs Wales were very memorable. I look at that pool stage and don't see too many games that would draw me in. How many of those games am I getting up at 3am for? I also don't think we need a round of 16 nor do I much like only the two best teams overall making it through to the next stage. That just simply rewards negative and cynical play.
At some stage we will have to address this because the World Cup will probably need to be expanded. I think we should do that when teams 19 and 20 like Namibia and Uruguay have shown that they can play competitive games.
What we really need is a system which allows teams like Georgia, USA and Romania the chance to play against the big boys more often. 3 big games every 4 years will not help these teams develop. I think one way to do this is to open up World Cup qualifying. Only the host nation should qualify automatically. All the other 19 spots should be filled up by qualifiers. In New Zealand we shouldn't have to play 10 qualifying games or anything but we should have to play at least 1 or 2. World Cup qualifiers are the only way to mandate teams to play the smaller nations unless there is a lot of money involved. I just don't think we should expand to 24 until we look after the first 20.