This is one of those topics like 9mm vs .45 ACP. Ask 10 people, get 10 different answers. Yes, steel (keep in mind this is very mild steel) would, in theory, be harder on the parts of your rifle, especially, I would think, small parts like the extractor. There is also the fact that steel is much less malleable than brass which could cause extraction issues because once the case expands during firing, it won't contract back as much as brass. I worked as a range officer for a while at a public range and saw alot more stuck steel cases in AR style rifles than brass cases. This was before Wolf and others came out with polymer coatings to replace the lacquer coating which seemed to be the big issue in AR's due to their tighter chambers. I believe that unless you shoot thousands of rounds of steel cased ammo through your gun, you won't do any appreciable damage to it. Ammo has become much more expensive in recent years and steel cased ammunition is a viable alternative to brass, especially for high volume shooters........Or you could take up reloading. I'm not so sure about the kitten thing.
"Those who hammer their guns into plows will plow for those who do not."