Clean up crews are in no way necessary, and it's true that they physically add to the bioload, but they also can be extremely beneficial. Clean up crews aren't just algae eaters like plecos (who are notorious water polluters). These include things like shrimp and bottom feeders which don't tend to add much to the bioload and also, clean the waste of the fish and rotting food. This is why clean up crews don't add much to the load, they clean their mess and the mess of others which equals up or far outdoes the mess they create. As long as you're diligent with cleaning and ensure there's not much rotting food in the tank, you'll be fine without them!