The argument that altitude training should be beneficial to lowland natives because most of the world's best come from high altitude is meaningless. Coming from genetic stock that has been living at altitude for 10s of thousands of years and being born and raised there yourself is entirely different than moving to altitude from New Jersey at age 22.
That said, there may very well be a benefit from training and/or living at altitude, it's just that the previous argument does nothing to prove that.
I tend to agree with some others who have posted that some of the fringe benefits of altitude (good training groups, nice weather, etc...) may be more important than the altitude itself.
Another possible fringe benefit (that I don't think has been mentioned yet) is that I feel that training at altitude promotes running by feel as opposed to being married to your stopwatch. I find myself more in touch with my breathing and other bodily feedback at altitude and I know that I don't look at my watch as much.