And I imagine a fair number of those who pirated also bought the game legally, either to have a digital backup or to run on a PC emulator.

So your main groups pirating a given game are:

1. people who never would’ve been customers
2. actual customers who own a copy and want it on PC as well
3. pirated first, and ended up buying the same later
4. actual lost sales

I imagine #4 is such a small percent of the total number of downloads that they probably help more than they hurt (e.g. word of mouth advertising).