Clipless has its benifits, mainly not having to worry about your feet shifting around on the pedals throguh the rough and rocky stuff - but if you are good with flats its not much of an issue.
You also have the ability to unweight the rear of the bike easier, but agian, if you have the proper technique down not much advantage over flats.
Getting out of clipless pedals instantly is no problem, I have unlipped and dabbed the instant I was losing it with clipless many times, however here the advantage goes to flats because you dont have to think about getting your foot clipped back in after the dab.
The only real benifit is putting down more power in your pedal strokes, but that only applies if you have proper pedalling technique down, and there is less benifit on a DH bike because it's hard to get clean full circular pedaling while standing up (but still an advantage if you get real good with clipless pedaling).
I use Crank Bros Mallets