In reply to mike kann:
I use a PAS (not the Metolius one though---the nylon one made by Sterling is cheaper and stronger) and think they have some advantages. None of them are decisive; you can always improvise something to work in an analogous fashion, but the instant availability of a an anchoring tether, whether you are roped up or not, seems to me to lead to more efficient procedures.
Leading sequence: When the leader reaches a stance an places the first anchor piece, he or she clips to that with the PAS and calls off belay (no need to haul up a big loop of rope for clipping). The second can then start getting ready to climb---readying the pack, tightening shoes, taking apart all but one piece of the anchor, and completely detaching the rope from the anchor. This can happen while the leader builds the rest of the anchor and arranges the tie-in. The leader's PAS is left clipped in for use in dismantling the anchor later as described for the second.
When the leader gets on belay, the second just has unclip their tether and remove the last anchor piece. When the second arrives at the belay, they clip in the PAS and are immediately off belay.
This is the technique recommended by that famous wanker Mark Twight in his book, Extreme Alpinism.
It was mentioned that the leader can just clove off to the first piece, which is true, but often the nature of the completed anchor will require that the leader undo that original connection and redo the rigging. This issue is avoided when the initial attachment is via the PAS. There is an analogous advantage for the second: no matter how the anchor was rigged with the rope and/or slings, it can be dismantled down to one piece while keeping the second always clipped in.
Following. Sometimes, the second will need both hands free to get out stuck gear. With todays thinner ropes and longer leads, trying to hang on tension from the belay can be a pain---rope stretch can put you way below where you need to be. Occasionally, because the route diagonals or goes over a ceiling, hanging on tension is not an option. Most of the time, the best approach is to place another piece and hang from it. Having a PAS ready to go for this purpose saves a lot of fiddling time.
Improvised aid. Sometimes, things don't go according to plan. The party is facing benightment, the crux pitch is soaking wet or maybe be too hard or the leader too tired, or a storm is approaching or has arrived. Aid climbing is called for, but the party is only carrying free-climbing gear. One improvised foot sling and a daisy or PAS makes it fairly routine to do this kind of impromptu aiding. All these things can be rigged without a PAS, but the party will have to have enough full-length runners and will be less efficient. If the situation breaks during a lead, it is not at all uncommon for the second without a PAS to be ill-equipped for such improvisations.
Prussiking. If prussiking is called for, either because the second has fallen and is hanging in space or because rappel ropes won't budge, the PAS is already installed, ready to form the harness connection, and adjustable enough to make the process efficient.
Multiple rappels. Both climbers will have to fashion some kind of tether for this situation, with the PAS it is already installed and ready to go. Plus, the PAS can be used to extend the rap device away from the belay loop. This is standard practice for those who install a back-up autoblock below the rappel device, but even if you don't use such a knot, extending the rappel device gives you a little more friction and allows you to employ both hands as braking hands, and this can help with rappels on modern thin ropes when it turns out that the braking device doesn't supply quite enough friction.
Mixed ground. When the climb has sections that are climbed unroped, an installed tether simplifies the transition back to roped climbing. In some cases, a pack or a second rope may have been put on in such a way that pins down all the runners carried, in which case the installed tether makes it faster and safer to get anchored.
Self-rescue. This isn't even remotely as much of a concern as the purveyors of self-rescue technique books would like everyone to believe, but in the very rare case when one has to do such things, having an installed adjustable tether will make many procedures easier.
Wankery. Ok, you aren't supposed to do this, but sometimes it is convenient to be able to untie and retie.
What I think many of the arguments miss is the full range of things made more efficient by an installed adjustable tether. Sure, you can improvise solutions to any one of the applications, but on long routes the team is going to be faster and more efficient it such improvisations are eliminated.
PS: Don't wrap the PAS around your waist, it get tangled in everything and takes a long time to deploy. Instead, gather the loops into the locker and clip to a harness loop. Deploying involves grabbing the locker, shaking the loops out, and clipping.