steel wont harm your gun any more than brass during the normal operation life of your firearm. the difference in wear is hardly measurable.
if your chamber isnt chrome lined you may experience extraction issues with some steel cases. steel cases have a much higher chance of case head seperation in ar15's so keep a broken shell extractor handy
there is more blowby into the bolt channel and firing pin channel so more frequent cleaning may be needed.
other than that there is nothing wrong it. i wouldnt rely on steel cased stuff for self defense purely because of the broken case issue
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent