Do you care about the value of your purchase? If you spend, say, $150 on a scientific textbook, and then you cannot show it in public because you would look like a pirate, you should care.

And that is the best case of a readable, complete book. Maybe you get only the first 500 pages, or the figures are cut off, or it is a degraded scan of a printed book.

The problem is of course much worse for collectible books for which even getting an official reprint or a different edition instead of the real thing would be a problem.