Hmmm. The simplest and most direct answer to this question is to get the best vfx house you can to do the work. I have had a number of vfx shots in the past that would have required green/blue screen. But because of newer technology, greater experience and in this case, money, the vfx house was able to pull a key from my frames with no problem. No green screen and most of these shots were handheld.
I am positive many vfx houses require: proper key to green screen lighting balance, preference of blue or green screen, locked-off camera frames, and a bunch of other technical stuff that will halt your production to a stand still.
The reasons are simple. More time is more money.
Anyway, these reasons are all valid and are all worthy for that perfect key, but it all depends on who and what the shot is.
HTH
Todd McMullen
Flip Flop Films
Austin
http://www.toddmcmullen.com