Im not really a great lover of zooms, never been but I would agree, the 24-70L, is definetely good enough for micro, thats for sure. Reading many tests, etc, many people swear on the 17-40. Ofcourse it all depends, you have to be lucky and get a good copy. I had to change my 24-70, twice, in order to get a good copy, theyre all differant, depending on the production-line. Friend of mine got a sample of the 17-40L and would you believe its far better then my 24-70L. Its a matter of luck. I bought a 21 mil, Leitz, first sample was crap, next sample was too good for words.
I know some will argue but the plain fact is and especially in digital photography, Primes is the way to go, theyre expensive but really they are so much better and very seldom lets you down. My own favourite zoom is the "new" white, 70-300L IS.USM. the sharpness and IS of this one is truly remarkable, a bit expensive perhaps but great.