As with everything on these forums everyone is all about rankings. Ok, for law school, it matters, but come on people, get your head out of Vault's ass. If you are working for any biglaw firm making market rate, it is not going to change your life that much if it is not a V100 firm.Future attorneys who have never worked are so arrogant. It's ridiculous. When choosing a firm, you should work where you feel that you would like to spend a career at. People leave all types of firms for all types of reasons and if that happens down the road, cross that bridge when you get there.
I think you're correct in stating that the vast majority won't make partner; however, think about the alternative of going to a mid-sized non-vault firm where you can make partner easier. Wouldn't that give you pretty good exit options as well?
My reasoning is that I have seen a lot of Partners at Vault firms who appear to have spent time at other smaller firms earlier in their career. So, they obviously were able to lateral in. That's my guess. Who really knows.