But, if your max "peak" power gets better, and your max "best" endurance capacity gets better, then even when you are in the in-between phase, you still are climbing better than before.

While I agree with that in general, my experience last year felt like playing a game of wack-a-mole: I'd do a fingerboard or campus/bouldering or PE phase and absolutely crushes each phase individually, but I had a hard time making the gain in each phase "stick".

My year sort of went like this:
I had a sport project (hard and consistent 4-5 bolt section) that required a combination of PE, finger strength and bouldering power. I started the year with good finger strength & realized I needed more PE to send the route. I trained PE for a few weeks, came back feeling great, but couldn't do the fingery start. I worked the finger strength back in, but then wouldn't be able to finish off the last bouldery move at the end of the hard climbing. I go back to bouldering, got strong & finally sent the route, taking early summer to early fall (thank god that wasn't all I did climbing wise), much longer than I thought it'd take.

At the end of the year, my redpoint capability moved from 13- to 13c ish, compared to 12d to 13- the year before where I just climbed mostly. Of course, it's hard to say I should have expected the same gain one year after a good climbing year, but I also became more of a one trick pony: I couldn't onsight as close to my redpoint grade and I couldn't redpoint my physical limit as easily (I didn't have the ability to "fight" thru the redpoint: it either feels easy or I fall off).

Devil's in the details on stuff like that, and personal interpretation of what various training terms mean, and how it applies in your specific situation. For example, when you were training PE, were you really training PE, or were you in reality doing more E than PE? And when you say that you got strong bouldering, is that really just P, or is it, in fact, closer to the PE training that you may have wanted to train earlier, but didn't?

None of it is standardized, and what I picture when you say PE may be very different than what you actually did for the PE phase, and maybe neither my interpretation of PE, nor yours, is strictly correct...

The more I mull over it and do specified training, the more I realize that nearly EVERYTHING is PE training.

Most boulder problems that are more than 3 moves (especially at my gym where the best problems are 8-10 moves long, on a 70 degree overhang) have some small component of endurance to them. Hangboard repeaters are more of an endurance exercise than a pure strength thing, too (though with next to no power component). And even when I do some sort of ARCing or >10 minute traverse, I still occasionally throw in a slightly difficult move.

For me (sorry, gonna spray), I have not seen an magic bullet solutions through periodization. I was stuck in the 12+ zone for years, gradually consolidated my technique and familiarity with diverse climbing styles, and then, in 2011, devoted more time to climbing than I ever did before. I broke into 13s, and by the time the year was over, I had redpointed 13a enduro, technical face, bouldery power, and crack routes. This was all just by climbing a LOT, including a full-time summer roadtrip. No structured training.

The next year, 2012, I began my first periodized training routine, and managed to redpoint one 13b, so I suppose that, technically this was a gain. Instead of climbing for a whole summer, I took most of the summer off, and then managed to redpoint one more hard 13a (without doing anymore training), which was more of a subtle technical route than one of pure fitness.

Right now, I am able to do nothing BUT hangboard, due to a broken foot, and am feeling really strong in the fingers and forearms. But because of the injury, I have pretty low expectations for what this spring will bring.

More seriously though, yes, I agree, everything that is more than couple moves long has some endurance component to it, and PE is a spectrum.

You are getting to the other thing that is often overlooked in these sorts of discussions, and that is not everyone has the ability to get on the rock as frequently as is optimal. Periodization training, while it may not be better than climbing regularly and a LOT, for someone who lives in a place with no true "off" season, and/or is able to travel to climbing destinations during the local off season, may be an answer for someone who isn't able to do so.

I know that for me personally the decision to try periodisation this winter had a lot to do with my anticipation of no outdoor climbing from mid-December on, for couple months. And since it is the first time I am trying it, the results are yet unknown.

Oh, the suspense! Don't you hate it when you build up the expectations, and things just fizzle out into nothing?

I guess that's why JT is always ranting about how climbing is, at the heart of it, an anaerobic sport. Which is true, but begs the question.

The trouble is that the key to climbing harder is just as much, if not more, about raising your anaerobic threshold than it is about increasing your capacity to climb once you're past that threshold. I mean, I can climb some types of 5.10 literally all day without getting pumped, and I can boulder V5. Right there, that allows me to send some 5.12 routes, even with next to no PE.

Most of the stronger climbers I know who can send 5.13 and up can climb a lot of 5.11 all day without getting pumped. So the real difference between them and me is that they can climb harder without getting pumped. Of course I'm not saying that it's all physiological. I'm sure a lot of it is superior technique, better strength-to-weight, etc. But in the end that all boils down to how hard you can climb while still mostly on this side of the aerobic/anaerobic threshold.