In reply to Vince McNally:
I've posted these thoughts elsewhere, but maybe repetition isn't a bad thing. Apologies for the length, but I think a simplistic approach to these issues is a really bad idea.
First of all, most of the time it isn't going to matter whether you are using an ATC or an assisted locking device. The ATC will, in some situations, lead to lower protection loads as a consequence of the rope running through the device under tension. If this doesn't happen, and for most climbing falls the rope doesn't slip through the belay, then the peak loads aren't going to be any different for the two types of devices. I know full-time climbers with an international resume who have used ATC's for their entire career and never had the rope slip through the device, so it is entirely possible to use an assisted locking device for years and years without encountering circumstances that would result in higher anchor loads than an ATC.
There are, however, relatively rare situations in which the rope would slip through an ATC and result in lower loads than would be obtained with an assisted locking device. Moreover, as ropes get thinner, the potential for slippage has increased, since the thinner ropes are harder to hang on to. This is even more true for half ropes, since the belayer may be handling them in a way that does not involve all four fingers grasping each strand. Moreover, ATC devices are marketed for ranges of rope sizes (excessive ranges in my opinion), and are typically not going to do as well on thinner ropes.
Taken together, I think these developments mean the historical absence of rope slippage cannot be viewed as a good future predictor. Thinner and thinner ropes used in barely adequate broad-range devices are going to result in more and more slippage.
Now slippage is supposed to be a good thing, the safety valve that lowers anchor loads. But I think it more likely that the load reducing effect of slippage will, on balance, be outweighed by its extremely unpredictable effects on the belayer. The potential for much longer drops, and with the danger of the falling leader hitting things along the way, seems to me at least as likely, and in fact more likely, then the loads on the gear being too high.
We U.S. old-timers grew up actually practicing dynamic belays with dropped weights. We learned that the impacts involved in really serious falls are much worse than you might imagine. Nowadays, the fact that people regularly experience difficulty controlling rappels with their ATC's, yet continue to imagine that they can deal with the far higher loads involved in holding a severe fall, illustrates an almost willful disregard for the potential loads in belaying. We also learned that if you don't have control in the initial milliseconds, you won't ever get it back, and then the leader is going for a very long, possibly terminal ride.
No one does this kind of belay practice any more, which means that most even highly experienced climbers have never had to hold a really severe fall. It is beyond peculiar that we speak of "competent belayers" in view of this almost universal lack of experience with really severe situations. Because of this, the question isn't really whether assisted devices are a good thing for new belayers, because almost everyone is a new belayer if a very severe fall occurs.
U.S. climbers abandoned dynamic belaying when ropes improved and testing made it clear that the enormous variation in individual performances made it questionable whether even those who had practiced a lot could perform successfully---and this is under test conditions, not real-life scenarios on the crag. Taken together, these experiences mean to me that if a really bad fall does happen, there is a pretty good chance that the ATC belayer will end up letting far more rope slip than is needed for load reduction and will end up endangering the falling leader and very possibly severely injuring themselves, in view of the fact that a minority of ATC belayers use gloves.
For these reasons, if you are using thin single ropes or even thinner half ropes, I think one of the assisted locking devices is, on balance, a safer alternative, even if it results in higher anchor loads a small fraction of the time. Moreover, all the newer devices have the ability to be used in "ATC mode," so if the protection is known to consist of brassies, ball nuts, and microcams, the team always has the option to keep rope slippage in the equation if the belayer is gloved and the protection system seems to merit it.