This is one of the dumbest things I have ever heard. I run Rocky helmet on Garchomp and Ferrothorn because knocking off 25% of an opponents health while taking damage I would have already taken is fucking AMAZING. I like how you are ignoring the fact that like 90% of OU right now(totally made that up), consists of Physical attackers. I love rocky helmet, and would use it over leftovers any day on Ferrothorn.
Are you forgetting about another huge threat called....Talonflame? Don't have the right check to that pokemon, and bam, your team lost 3-4 pokemon to a CB 252 atk STAB Brave Bird. Throw in a garchomp, and you have a dead bird. It's not SOLELY for Mega Kanga that we use rocky helmet, but we do get the warm fuzzies when one pops up.
Yeah, it is used on Garchomp and Ferrothorn. Absolutely. Though, again, it is not the "only good choice" on them. And it is only viable on them because they have the bulk to weather multiple hits, and they have Rough Skin / Iron Barbs. As I pointed out, you don't, and never would, run it on either of them to handle a single Pokémon.
The post I was quoting was arguing that it would be a good move to make it a go-to item on everyone. That was why my points about it on the average Pokémon came up. It is not a good item, for pretty much any Pokémon, except to use that Pokémon for killing Mega-Kangaskhan via attrition. I think I worded it poorly, sorry about that.