I love Steam sales but I feel pain for the developers.
The number of AAA titles in my steam library that I've barely played is pretty high. I could go without buying any games for at least a year before I would consider buying another one.
My time for playing games has become so small that I'm well behind the release curve, so games that I do want to play are frequently < $15.
And... sometimes I don't get around to playing those games, because something more interesting comes along before I get around to it.
So yes, the developer isn't making as much per user, but I believe the increased volume would often make up for it.