I have SPD's love them for all types of riding. (granted I've never done any REAL Downhill yet...)
Makes controlling the bike really easy, I can bunny hop and "throw" the bike with ease, and higher than I could without.

I was doing 4-5 drops the other day with my Kona, to test my suspension, (I was jumping rain water run-off ditches and etc.[I had an audiance, so I had to do something half way cool, lol]) My SPD's didn't hamper my performance any way. I almost lost it once, my suspension bounced back to quickly (dang rebound, I can't get it right) and threw my into a catwalk that I could recover from (was going towards a bench) I was able to unclip and put my foot down to stop myself with no problem or extra effort.

bottom line, is I don't see why people make a huge deal out of not being able to unclip, I even have my pedals tension rather snug, and I have 0 problems getting in or out.
The learning curve for me for clipless was painless also. It took a few roots, and a couple curves at the beginning of the trail to adjust to them. In all, only 5-10 minutes and I was set.

-I have no problem hoping a bike with flats either (my trek now has those wierd flats/SPD combo things that came on the Kona.)

I also understand that it's personal preferance as well.