One thing that has not been directly mentioned is that a good photographer will envision the amount of DOF he/she wants in a shot and then adjust the aperture to match based on the focal length of the lens, the distance to the subject and the distance from the subject to the background. As the distance variables change, so must your aperture to keep the same DOF
Thanks, I didn't think about this point, keeping a consistent dof across the lens zoom capabilty certainly is a plus for a professional appeal.
Also a big thank you to all posters explaining the pro and cons to me w/o any "just get the best" attitude, to summarize my current understanding f2.8 zooms are for
* keeping a constant dof
* pro-looking bokeh
* faster shutter speed for fast motion with zoom
* reserve in available light situations, though as far as noise goes 5d2+f2.8 = 6d+f4
* sharpness because f2.8 lens stopped down > f4 lens wide open
* more reliable af - though I wonder if this is valid for the 5d2/6d af? Anyone?
All these points make me tend to get a Tamron 24-70 lens for the 6d after all since my lenses don't pay for themselves by pro shooting, I will shoot somewhat stopped down often and 100% crop sharpness is nice to have, but not critical to me... and the plain matter of the fact is that I could get 2 (two) 6d+24-70vc for 1 (one) 5d3+24-70ii :-o