I don't really care if the 9th best team in the West doesn't make it, but I think taking the top 8 in each conference is better for regional match ups that drive fan interest.
MLS learned that the hard way. The west has always been stronger, and for a few years they countered that by allowing the last playoff spots to be filled from either conference. It meant that RSL went to the championship game by beating all the eastern teams. It was a fun run for sure, but it was just a weird format.