Actually, most bird shooters and many nature shooters will use a flash with a 400, 500 and 600mm Supertele, with or without TC. I'm pretty sure that's most of the super-tele market. I'll often use one with my 400 and 1.7x TC (with a "Better Beamer" attached) if for no other reason than getting a catchlight in the eye if the bird's position relative to the sun isn't right. Especially with the 400mm, which goes to f/2.8, even with a TC you can often use a flash as a fill flash and get better results than not lighting at all. A lot depends on your subject distance, which for smaller animals and birds is often within the range of the flash without an extender, though they're well-worth having.
http://www.birdsasart.com/accs.html#BEAMER
http://www.luminous-landscape.com/reviews/better_beamer.shtml