I wouldn't say torrents themselves are "evil" or that unauthorised copying is either (as the OP has pointed out, it will lead to some extra sales). But demos can have issues too (may not be updated to fix bugs or performance issues and can give a misleading indication of gameplay) and they require support and download bandwidth from the developer.
It's worth noting that GOG now offers a
30-day guarantee if a game fails to work on a system, and most games there have several user reviews.
