And just being rubber should automatically give you the ability to touch electric energy (remember that everything passed through Enel/Eneru's body)? Luffy will put goop on his hands, and voilą, he can beat up Smoker too!
If it's a simple matter of putting stuff on your hands to beat up elemental ability users, where is the fun in that?
(My dad could beat Luffy with his hands on his back. He hate the "Scare crazy people to tears-fruit" (OMG, that was so awesome..! But it's a long story, and this is a One Piece-forum)