Training at altitude?

I was thinking today about a paper I wrote recently that delt with the benifits of training at altitude. Besides enabling the blood to carry more oxygen, what does training at altitude do for an athlete in preparation for a fight or competition? The question I tried to understand but was unable to figure out was: what is the proportional viture of training at a certain elevation? Training at Big Bear is more effective than training in Los Angles or so it has been assumed. Obviously training at 5,000-7,000ft is great when coming from sea level, but what if you either wanted to do more or you're already living at that elevation? You cant go to an extreme because of HAPE, but couldn't there be a value of train at 10,000-12,000ft?