Also, when you get into these extremes, you need to be shooting in cold, clean alpine air, or humidity and other environmental conditions will wreck your image.
Depends on what your shooting. I have no need to shoot wildlife at 1200mm, 600mm is actually more than plenty. Sometimes deer and the like get so close out of curiosity that I can't even get a shot.
With waders and other larger, shy birds, that might be a problem. Depends on whether where you live is humid or not. I live in Colorado. We've had some hot years, but most of the time it's not blistering (80 degrees or cooler) and it's pretty dry up here in general. The only time I've had problems with water vapor being a problem was when the temperature was near or over 100 degrees...then it's a problem even at 600mm.
The real benefit of 1200mm comes into play when your photographing small birds. With just the 600mm, even at a relatively close distance, with a full frame, they are quite small. Moving to 1200mm with passerines isn't because they are far away...it's just to increase their size relative to the frame, but your still relatively close. More than close enough that water vapor in the air isn't going to be a problem between lens and subject (it might wreck your background boke...but that's a whole different deal.)