I would like to point out that you are assuming the person using these photos is able to post-process in such a way as to make them more presentable for their p

I would like to point out that you are assuming the person using these photos is able to post-process in such a way as to make them more presentable for their purpose. For many people this is not the case - they don't actually know how to make the images better. I would argue that for them, these kinds of sites are a great benefit and make their lives much easier (and cheaper). Secondly, using one photo from each is hardly representative of the whole. Many of the simple stock photos that are found can be rather cheesy.