50mm 1.2 DOF at 2m = 0.11 m
70mm 2.8 DOF at 2m = 0.13 m
Not a huge difference IMHO.
^ I totally agree that the DOF can be the same given optimal location and the ability to move around and frame.
Unfortunately, for me, I am never in a studio and often don't have the ability to frame many shots other than my 'foot zooming' and some post process cropping.
What I mean is while I think a zoom would be convenient, I like my thin DOF as well as my working distance.
My 50L is quickly becoming my most used lens but I don't always have the ability to back up, as I would need to zoomed to 70mm at f2.8