The short answer to your question, RaikouLover, is yes. Losing your most powerful STAB move can be worth it when your STAB is of a type that sucks ass.
The whole problem with the "Return does more unless the move is 4x effective" argument ignores the fact that nothing that would come in to a Slaking (especially a lead Slaking like this set is designed to be) is going to take neutral damage from it. When 5 of the top 10 most common pokemon in the game laugh at your precious STAB move, you might want to rethink your move selection. Foregoing STAB is not an insane option in this case because: 1) Slaking has more than enough Atk to cover the minimal loss of base power, 2) Normal is a shitty attacking type in D/P. There is no arguing against this.
Although I do feel that Return is much more viable than ThunderPunch and should probably go on the set anyways, I've been using this set for a little while and I have yet to use my Normal move. It's just not a great move anymore and keeping it as a mandatory option is really limiting Slaking's potential coverage for a couple of extra points of Base Power that won't even come in handy most of the time because it will just be divided by 2.