I use Lightstock, Adobe Stock & Shutterstock as well. I sell the most variety of images on Lightstock. Adobe Stock sales tend to come from my images that are clearly faith-based (a few photoshoots I’ve done for Lenspiration assignments have sold REALLY well on Adobe Stock). Shutterstock yields less sales for me. And I agree, it’s probably not worth taking the time and effort to submit your photos to somewhere for free when you could do the same and earn something off it.
Another note, stock photography is certainly not a “get rich quick” method of photography. 🙂 However it’s a nice way to have some earnings come in on the side and it’s nice that it doesn’t need to be maintained once you’ve uploaded your photos. But at least for me, it took quite a while before I really started seeing sales happen consistently.