I beg to differ. The cage of my front mech is about 2mm above the outer circumference of the bash ring, and I have no problems with bad shifting or chain rub.
If you have a look at the inside profile of a 3x FD you'll see wider 'bits' where the chain runs on each of the three chainrings, so you can use the whole range of the rear cassette without chain rub. In between these are narrower 'bits' so that the chain is guided accurately and quickly between rings during a change.
So, if you then take the big ring off, and lower the mech, the remaining two rings are still in the same place but the FD has moved in relation to them, so the chain is now running in the narrower bits when on the rings, and you're shifting on the wide bits.
Will it still shift - yes - its not rocket science as all the FD really does is yank the chain off one ring and send it in the general direction of another, but all that clever design work and intricately shaped ramps and bumps inside the FD cage that's there to make your shifting experience as smooth as silk, is wasted. Try putting your FD back where it was, you may be surprised...
If you're still not convinced, ask yourself this... If my FD shifted fine between the granny and middle when I was on a triple, why would I need to move anything when I simply don't shift into the big ring anymore?