Steam sales are AWESOME. Certainly for the consumer, anyway I can't imagine Steam sales hurt publishers -- it leads to many more sales than they would have ever got before. I took the plunge with Pinball FX2 in the last Steam sale, which means I am another person with Pinball FX2 on my computer, which means I am another person who may buy more tables in the future.
I also worried that TPA's in-game-only purchases might prevent TPA from taking part in Steam sales. I immediately bought Season One already, but it would be nice if I could get season 2 at Christmas at 50% off.
Steam sales are a huge boost to sales. You can read this article for example.
http://www.gamasutra.com/view/news/174587/
I have personally bought so many games just because they were on sale and "cheap". I have about 15 big budget games and even more indie games just sitting in my Steam library that I have never played even once. Quite a few of my friends have a similar situation, so I can safely say it is quite common.
FS is just crazy if they don't do all they can to get into the normal Steam purchase option and participate to these Steam Sales events.