1. Top firms recruit more regularly and aggressively at top schools. Check out job placement stats for proof of that fact. It's much harder to go work for a F500 company with a top 100 MBA than it is with a top 10 MBA. Plus lower-ranked schools tend to be more regional, while higher-ranked schools are more likely to be able to place you anywhere in the world.
2. Aside from differences between state vs. private schools, tuition differences between top 50 programs are fairly negligible and are more than offset by expected earnings over the course of a career. Therefore it makes sense to go to the best school you can get into that fits your goals, personality, desired location, etc.
emont, thank you for the thoughts, I also had the same question and you have been able to answer it through this post.