I couldn't find any reliable source except a few indie developers who say "yes, it does" (their games don't sell well) and others who say "no, it helped me promote my game" (their games sell well)
but to be honest how many games of big gaming companies have you bought where you had to find out the game actually sucks? would that happen with indie games? not in the slightest they have
no money to promote their games thus accidently buying indie games that suck doesn't happen as often (and because of the few $ you might even not care).
on the other hand, have you bought an indie game after you pirated it? I couldn't come up with a single occasion as I really do buy every single indie game I played even if in the end just for 2-3 hours.
and last but not least game size to download, big games usually pack 7-9 GB dvd downloads, an indie game isn't more than 100-300MB, one takes a few seconds and the other (depending on connection and method from 3 hours to several days) and the moment you played through the game (indie games are usually short) you might not even want to buy it any more.
so what do you think? does piracy hurt indie developers?