Personally I use these services like a public library too. I buy most of the books I'm sure I want but I use public libraries and download sites alike to explore more. Once I actually dive into a book I buy it but many I don't.

Of course this depends on a honour system as you could easily download everything. But books are cheap for the amount of time you spend on them and it's a nice thing to buy.

Note: for technical books that equation can be very different and I could be more likely to download yes. Though i don't usually consume technical content in book form anyway. I tend to explore by doing and learning as I go.