It's true, the microworms are far too small for most adult fish, who probably won't even notice the worms are in the tank and floating in the water.
That being said, they are good for young fry. Also, very small fish, fish with small mouths and bottom feeders like kuhli loaches and pygmy cories will still savor them once they've collected in the substrate. Plus, I see you have an RCS tank -- those guys LOVE microworms.