Wow, I am flabbergasted. But then again I should have known, after all I love mathematics (really).
James Brandon on DPS talks about the bad sides (and the laziness I may add) of the focus-recompose method.
What caught my attention is that it is a method that I extensively used this summer in Italy; I even blogged about it.
So, yes, the math is simple and I am convinced: unless I have a small aperture (and thus a great depth of field), I shall no longer use the focus-recompose method.