I specifically use it because while I'm committed to writing free (as in freedom software), my downstream users may not be. I'd rather they didn't have more options than I do if they decide to compete against me. Free software empowers the downstream user, so I kind of think of it a bit like surfing. You potentially have all this extra energy, but that energy can crush you if you don't walk a very tight line. As the upstream author you have a bit of an advantage, but it's easy to squander it if you don't think carefully about what you are doing. I'm always a bit perplexed when people write free software but intentionally avoid copyleft. There are lots of places where permissive licenses are the right way to go, but also a lot of places where your are just creating an uneven playing field with yourself in the worst position.