I don't know what the percentages here are, but you can bet there are people here who play, yes, even Silent Hunter games without buying. When SH4 came out with its secret bootleg ID system, where illegal copies would have a crew with no eyes and crew chanting mysterious numbers, we had a LOT of people posting about it.
You want to know the best info about PC games an piracy, here is an article by noted tech author
Koroush Ghazi, PC Game Piracy Examined
Hopefully there are some who have long enough attention spans to read this all the way through without resorting to a session of HALO. It covers Steam, Starforce, the disparity between PC and console sales, and more. You can learn a lot from it.