High mileage has the rare distinction of being overrated and underrated at the same time. I'll explain... You can run very fast at say 50 miles a week if you run quality miles, and in the short term you'll problaby run faster than if you ran 100 miles a week. So in that sense, why do high mileage?
The answer is of course that down the road (3 months, 6 months, a year, 2 years?) high mileage will help you run faster, but its no EPO. You're probably not gonna go from a 15:15 guy to a 14:15 guy just by upping your mileage from say 50 to 95.
Training methods in general are all overrated. One should worry more about shaving seconds off their time by racing their ass off then about how much mileage to run, what % of their VO2 max should they run it at, and when to taper.
Finally, a knowledeable observer may ask, "A_Goofy_Runner I see your point, but where did you come up with such a theory?" And to that I reply, I made it all up based on what I have witnessed in my short and mediocre (at best) career. It might be true it might not, but it sounds good.