I see your point about the leverage, but for me I can't really tell a difference in performance. There are a couple of reasons why I do it. First, tire selection. There are just a lot more choices in 700c than 27" now. Second, I have very short legs and generally ride 49 cm bikes with a 53 cm top tube. The 4mm difference in height can sometimes mean being able to straddle the top tube comfortably or not. Third, a bigger selection of wheels I can buy. I like to take older 6 speeds and make them 9 speeds. This sometimes means just buying the frame. I can find some very nice built 700c 9 speed wheels, in order to get 27" wheels, I would need to buy the components separately, making the price much more expensive. At least, I haven't seen any 27" 9 speed cassette wheels, only 27" freewheel wheels.