In reply to Rich Tyler:
My take is that most of the time the difference between locking devices and tube-style devices is moot. The rope has to slip and run through the tube-style devices in order to realize their theoretical load-limiting advantages, and this doesn't seem to happen all that much, probably because the total amount of friction in a typical climbing system prevents loading the belayer at or beyond the slipping threshold. The net result is that most of the time Grigris and tube-style devices result in the same static belay, so use whatever you want.
I don't think the so-called "dynamic belaying" techniques used by sport climbers are relevant to multipitch trad climbing; there are way too many circumstances where the precisely-timed jump needed to obtain load reduction just isn't going to happen, and the CAI belaying tests indicate that simple lifting of the belayer has a minor effect on peak load---nothing like the "order of magnitude" suggested earlier.
There is, in principle, a way to get a bit of load reduction from slippage through a tube-style device without the rope running through the belayer's brake hand: this is what the CAI testers called the "inertial phase" of the belay, when the rope slipping through the plate pulls the belayer's hand up to the plate. (Rope only slips through the belayer's hand in the next phase, when the hand can't move anymore).
However, the inertial phase reduction, which is might be similar to what you might get from a screamer (and so depends on the actual length of the fall and not the fall factor), is often, at least from my observations, precluded because belayers tend to keep their brake hand very close to the belay plate.
So if Grigris and tubes effectively, in the field, have the same loading effects almost all the time, what about the relatively rare circumstances when a combination of fall severity and low systemic friction cause the rope to run through the belayer's hands? This certainly has a "safety valve" effect that isn't available from a Grigri, but it comes at the expense of a potential loss of control of the belay, or at least the burning of the belayer's hands, since in my experience only a minority of belayers use gloves. As ropes get thinner and thinner, the chances of loss of control and burning increase.
Put all this together and I think that most of the time it doesn't matter, but when it might matter you're still better off with some kind of locking device, especially if you aren't going to wear gloves for belaying.
Although Grigri's don't work on half ropes, we now have The Mammut Smart Alpine, the Climbing Technology Alpine Up, and the Edelrid Micro and Mega Juls, all of which provide (assisted) locking on half and twin ropes, and so the discussion is still relevant for these other systems even if the Grigri can't accommodate them. In fact, it is even more relevant because the thinner ropes used in double-rope systems are harder to hang on to.