Need refutation of Rocky Helmet

A day ago I met somebody that used Rocky Helmet on Scizor.

When I politely reccomended to replace the item I was told that Rocky Helmet was a good addition due to providing +-900 HP(Flat HP gain + % HP gain) and dealing damage.

I can see this player did the math and got informed before playing, even if that led to wrong conclusions.

I may quote stats info on why Razor Claw/Weakness Policy/Attack Weight are better options, but I doubt that would change this player's decision, given this player has stats info on Rocky Helmet.

Of course, I could just let this player play a unusual build, but there is a problem.

I fear that if I don't know the exact reason why those items are better than Rocky Helmet, my build selection might make a bad impression on my team members, and I don't want that to happen.

I don't want my team members to start asking me to include Rocky Helmet on every Mon.

So I wish to know why Rocky Helmet isn't the best item on Scizor. Any tips?

In fact, that raises several questions. Why is Rocky Helmet considered the worst item available?

As you may see, I'm not a very knowledgeable player, I just use the reccomended Unite.db builds. But I like to learn, and this has caught my attention.