I have slept with a feather mattress (actually, it is a tick, not a mattress, what we call 'topper') for years and years. I have never taken off the topper in the summer, or any other time, except when J was an infant and I was worried about SIDS.

I also have a feather comforter that stays, like Barbara, at the foot of my bed in the really hot weather.

Feather/down is an insulator...both ways...it is kind of like people that shave their dogs/cats in the summer. That fur has an insulating effect both in the cold AND the heat. I think the feather mattress serves a purpose in the summer.