Sorry Brit, but no - it's exactly the other way around (unless we're talking a major hyped title, like StarCraft II and like).
If you try out Impulse and Steam you'll see first-hand why these systems generate extra sales. I've done impulse-buys on both platforms after watching the review videos - the "Buy now" link is never far away
Steam, being my personal favorite, is more than a distribution system. It's a online gaming community, including Friend's list, ingame chat, ingame awards system and more. This means, you'll see which games your friends are playing and how they're doing (awards, virtual medals ), enticing you to buy the same games to play with them. It's very much a "X-Box Live" community for non-console gamers.