Upload & Sell: On
I'll happily use a doubler when I deem it useful, and I am perfectly happy with the results.
Here's one; taken from the top of a tiny boat, bobbing in the waves, very flat, poor light, hand-held. This is a very significant crop (down to about a quarter linear measure); the birdie was a long, long way away, having lunch of another birdie. I didn't have my long lens, and couldn't have controlled it under the circumstances anyway.
Canon 7D, wide open (f/5.6), 70-200 f/2.8 IS II at 200 (so 400mm focal length on the shot), 1/1000, ISO 1600: