I only have one book for "sale" at Smashwords and I've had six times as many downloads since Sunday as I've had since I published it. (Of course, I'm new and an unknown quantity, so I didn't have all that many to begin with.) Since my other works (so far) are always free the only way I can see this paying off is if some readers post positive reviews. Either way, I don't think I've lost anything. I suspect most of the people who were motivated to download would probably not have been buyers.
Interestingly, my free books have both experienced big boosts in downloads, so I assume people have encountered them while going through the promotion lists. That's not bad, either.