That's not true. The GPL wouldn't be unnecessary, it would cease to work. The GPL relies just as much on copyright laws to work as does proprietary software. Something that is disallowed with the GPL is taking the source code, making a modification, compiling it, and then only distributing the binary. There is nothing that prevents this but copyright law.

A correct statement would be that the BSD or MIT licence would be unnecessary (except for their clauses that you have to include the copyright notice with your modified program).