I daresay that Dan Atherton breaking his knee prompted quite a few DH'ers to buy neck braces too.
I reckon Breaking his neck probably had more of an influence....
When I last raced DH a couple of years ago now, Leatts were just starting to be seen at races, Having gone to Aston a couple of times in the last few months they seem a bit more common, maybe 1 in every 7-8 riders in one Vs 1 in about 30-40 a couple of years ago so I guess they're more accepted...
You look at some of the debates on SDH and similar sites though they still seem to kick up quite a fuss still, some claim they are merly a fashion accessory and are more likely to break your collar bone than save your neck, where as others view them as essential protective wear and won't even consider stradling a DH bike without one on, I have to say I'm yet to be convinced either way on Neck braces myself and they are still really bloody expensive and the substantiation of them still seems a bit tenuous, but the type of injury they claim to address is probably the most significnt you could sustain (short of death) on a push bike...
As for other more general Body armour, I think if you are going to push yourself riding DH then you really need some Torso and definately Spine protection, I use Knee pads alot not just for DH, but have never really bothered with Elbow pads (not sure why, it's not like my elbows are any less likely to get smashed up in a crash I suppose) and I only use my FF for DH days....
Ultimately it all coms down to risk perception and assessment, everything in life carries risks; DH racing and riding you are choosing to tackle lots of difficult and high risk obsticals at speed but only for relatively short periods (say 1.5 - 5 minutes depending on the run), on my normal XC loop I'll ride for several hours at a time, but the risk factor for 90% of that is relatively low and for the tastier stuff you'll often ride a bit more "Feet up" rather than really attack in case a stack means not getting home, so it's XC piss pot and maybe knee pads, in fact I'd go so far as to say getting storm troopered up for a longer ride probably increases the risk of a stack, you'll get hot, dehydrated and tired quicker, thus making a mistake more likely... IMO...