I've had experience both selling just a few pins, and trying to sell 100+ pins on ebay (and not talking about hidden mickey lots or such).
When I'm selling just a few pins, like 5 or 10, I'll usually take a pic of the fronts and backs if I can.
But when I have more than 100 pins to sell, it gets very tedious and makes selling really exhausting. In fact, my family and I have gone through the entire process of looking up pins on pinpics, taking front and back pictures, and then putting them all on for decent prices... only to sell 1 or two in a week. When you do that much time consuming work, it can honestly be very discouraging to try and sell pins on eBay.
So now I'm pretty much considering using stock photos for pins unless they are rarer or I'm listing them for a bit of a higher price than the run of the mill sort of pins. I know I don't much care for stock photos, but I also know I personally won't pass up a pin just because there is a stock photo if I really want it. If someone asked for actual pics of the pin, I would have no problem sending them some asap.
Hope this helps a bit to maybe understand why some just use stock photos. I know that may not be the case for everyone, but after trying to sell volumes of pins and not always getting the hoped for results, I definitely have a bit more understanding as to why some sellers use them