Some people care, some don't. I have yet to see any proof that steel cased ammo wears chambers or extractors any different than brass. Some people have claimed that the laquer coated steel cased ammo can gum up the chamber and cause FTE's in AR's but tons of people use it with no problems. I have never seen that happen. Probably highly unlikely to happen with a properly cleaned rifle. Personally, I save the less expensive steel cased ammo for the AK and use brass in the AR. I know that HunterCO and Gungrinder use steel cased ammo in their AR's so hopefully they will chime in with their experiences...

ETA: I have also heard that steel cased rounds don't seal as tight in the chamber and can contribute to a little extra required cleaning, maybe others here will speak to that as well...