I would say the dust cover is usefull. But the FA I wonder how often they really are used outside of full on battle? The slicksides from DPMS are pretty nice. I believe they are called the "Sportical" or something like that. I was going to get one but thought it really just didn't look like an AR without the FA and dust cover/shield. Sorry didn't see the last question.

The dust cover is a spring loaded door that covers the ejection port when not in use.