In reply to Tom Ripley Mountain Guide:
I think many belay anchors are suitable for a direct belay, though I agree that judgement is called for. And let's not forget that if the purportedly braced belayer is pulled off their stance, the anchor that was supposedly being protected will be subjected to a higher load than it would have been with a direct belay.
Personally, I usually belay off the anchor, but not in the usual sense. I belay off the rope tie-in loop at my harness, and always adjust the tie-in so that it is taut to the anchor. Any load that comes on the belay device is transmitted by the tie-in to the anchor. This still allows for bracing to reduce some of the load (because the tie-in stretches). If for some reason slack builds up and the second takes a "leader fall," the belay is partially protected by the stretching of the tie-in strand, rather than having to take the full impact directly.
But the main reason I do this is I hate being belayed with guide plates and have no desire to inflict the miserable experience on my partners. As a second you are always being pulled, and if communication is bad you can't step down. If you are climbing over roofs or are on traverses, the inability to reverse a move might make you fall off.
Moreover, there are certain circumstances, such as when the anchor is at the back of a corner, when the belay plate, when loaded, will be forced against the rock in a way that will make it impossible to unload, and then lowering, if that is called for, can only be accomplished by a full-on belay escape that unweights the plate and removes it from the system. (I know of at least one situation where this has occurred.)
Speaking of lowering, it is clear from numerous accident reports that a significant number of users of the plates do not know how to safely lower a climber, with the result that climbers are sometimes left hanging or, more typically, are dropped. The shenanigans required to unload a plate and safely lower are truly hilarious and surely belie the superficial appearance of simplicity the plates afford.
I've also noticed a form of tunnel vision among those trained to belay directly off the anchor with plates. In constructing an anchor, they look for locations suitable for the direct belay, and I've personally witnessed some questionable anchors constructed to respond to the demands of a direct belay when far better options were available that were, not, however, at all suitable for direct belaying.
Some people say the guide plates are better for belaying two followers simultaneously. This is just plain false; you can do a much better job off the harness belaying in the usual fashion, especially if one follower is moving up while the other needs to step down.
The plates have a devoted following among alpine and big wall climbers whose interest in speed has the belayer seriously multi-tasking at the belay. Just how good an idea it is for the belayer to be repeatedly dropping the ropes to do other things is a matter for a separate debate, but forgetting those issues, there is no good argument for using the plates in that way on smaller climbs where the few moments gained by the belayer eating or changing clothes while belaying are just not significant.
The final joke seems to be on the guides themselves. Quite a few of them develop elbow tendonitis from continually hauling ropes through the plates, which impose more resistance than the usual configuration at harness level.