Hull cleaning is helpful in preventing the spread of invasive species only if the hull is cleaned before you transport the species in question to a new environment. Obviously, once the vessel is here with her unwanted and uninvited guests, there is no way to get rid of them, short of actually going dock to dock and boat to boat and removing the organisms by hand and disposing of them (which does happen, believe me.)

Regarding your point about making anti fouling paint as toxic as possible; that has been tried and that has failed. There's a reason most of the civilized world has signed the treaty banning tributyl tin in anti fouling paints.

You are correct however, there are many reasons to have your hull cleaned relatively frequently. Not the least of which is if you are going to take your boat out of the area, you reduce the chance of introducing an invasive species to that area.