Since melmetal doesn't have any 4x weaknesses(meaning no immediate KOs) then if the opponent uses like a coverage move/a strong STAB move it couldn't use that move next turn, putting you at an advantage? Also melmetal doesn't benefit from the defense boost either.
So is it worth it or not?
It depends. Are you trying to stall out their Pokemon so they can't use coverage, or do you want to raise your already amazing defense stat. If it were me I prefer g max meltdown because if they don't 1 shot you they give you a free turn to set up or steamroll their Pokemon. If it raised special defense it would be better because of Melmetals special defense being relatively low.