I never understood the logic behind this one or where it's origin resides. I abide, but always thought it should be the other way around.
I understand that for the uphill rider, restarting would be annoying, but I've never been a huge fan of slamming on the brakes while on a downhill (good way to endo). On short notice (around a corner, maybe) the uphill rider will have a much easier time stopping to avoid a collision. Easy enough to restart going uphill.
I think it depends on the downhill section.
If it's a crazy steep section that is going to put the downhill rider a risk if they go to slow or stop, then I would agree the uphill rider should probably just suck it up and yield (and, yes, it does suck to have to re-start on a hill, but chances are anyone would agree that it's better to re-start on a climb than have to be carried out on a descent.) There are definitely specific descents I can think of where I'll yield to the descending rider.
But if it's just a regular singletrack climb where the descending rider isn't at risk, then I think they should yield. They can easily rebuild speed. The climber has to rebuild momentum, which is harder (especially on his/her knees!)
One exception to both -- and one of my biggest and only pet peeves in riding -- would be level of committiment. If I'm halfway up a hill and another rider appears at the top -- regardless of the danger in the descent or what have you -- he or she should
f*cking wait until I'm clear if there is only one line. And sames goes for me if I appear at the bottom when he or she is halfway down. To quote Mr. Peter Griffin, this really grinds my gears. This happens so often and it's just annoying. It's very rare that I see someone stop and wait in this situation and it's always annoying to the rider who is already on that section of trail. I know it's difficult enough to determine who has the right of way on many sections of trail, but this is just common sense and it's the single most frustrating thing I see when it comes to interactions with other riders. Why create a potentially dangerous situation? I always wait in these scenarios only because I hold out hope that kharma will come back to me. But it rarely does. And it doesn't seem to matter whether the opposing rider is a full on XC hammer, a downhiller, a newbie, or anything in between. Very, very rarely does anyone seem to put themselves in the other rider's position in these cases. And I don't buy the notion that it's because they didn't see them. That may apply to a tenth of all situations like this (if even that many.) Many times, in my experience, it seems like the rider who should wait views the other as a potential audience to see how awesome he/she can ride. And that;s just bullsh*t. Seriously, I should probably just start yelling that I think they're awesome already and don't need to prove it. Maybe then they'll wait.