Well there seem to be two opinions. One only best teams should be able to participate in the WC and only best teams during the WC ending up in finals. Second WC is a big event and more teams the better as the other teams also get a chance to compete and get noticed, and it will help popularize cricket. Then there are commercial interests too.
I am personally in favor of 10 teams WC, as WC should be a competition between the best teams. But other teams should get a chance hence the qualifying part. That way WC will remain competitive, in the last cricket WC the results were expected and top 8 teams ended up in the quarter finals anyway. Both Ponting and Smith complained, Ponting saying they are finding it hard to get momentum.
Not just that I think it should become even more competitive, and teams go straight to semi-finals and play another six matches to decide who'd be in the finals. That way the luck part is taken out substantially, with Quarter finals even the 8th team has a chance to beat no3 team, and end up in the semi finals.
There is no harm in trying it I suppose. There is always the chance to return to 12+ teams in the next WC. Maybe another tournament can be arranged where all teams can participate. At the moment I would say Test WC is even bigger priority.