Well, you know, AS by itself is the cat's meow. You don't need anything else and you certainly don't need to spend another 11 quid per litre for pumice.
Remember Visuvius, the volcano that wiped out Pompei? Well that's what killed everyone, inhalation of the pyroclastic flow of hot pumice.
If you can afford AS then get it because as Ed says, it does a really good job on it's own. It can't be improved upon significantly by the addition of PS. If you are constantly changing scapes or stripping down the tank and putting new AS in then you don't care about the long term nutrient store. Barr is forever saying that we need to understand the science of the products that we use. PS sells like hot cakes evidently, and why? Maybe people just can't be bothered to understand the science. You see, I still remember getting repeatedly burned by buying Dupla Drops for a long time until Sears & Conlin figured out that it was just basically KNO3, Epsom Salts and Traces. It was like being unplugged from the Matrix, so now I figure products have to prove their value. The higher the price the more stringent are the requirements of that proof.
For people who don't want to be bothered with home grown solutions and can afford it then that's their choice. I'm the same in some respects, I can't be bothered making my own CO2 diffuser so I'll buy one. But for substrates there is a choice for those who don't have the means. The treated Akadama plus peat as described by JamesC in his sticky is as good an alternative as any, especially if you plan to dose the water column. It's important to note that Ed sees a marked improvement when using AS because his column dosing is sporadic and perhaps a bit lean - for good reasons. On the other hand I see much, much less of an improvement because my column dosing is "over the top" to say the least.
As you point out, over time the organic waste breaks down in the sediment and releases nutrients back to the roots. I can't quantify whether it's ever as high as the original amount or not. I'm sure it depends on a lot of factors.
There are a lot of factors which determine the health and growth of your plants besides the substrate. Here is a classic illustration. Look at the photos below. The first photo is of P. stelleta in 100% Aquasoil Amazonia (No PS). The second photo is P. stelleta in a mixture of inert gravel and a little bit of Ecocomplete. These plants are in the same tank but the sediments are segregated. Can you tell the difference? No, you can't. In this case water column dosing and extreme CO2 override everything else, including substrate performance. I could take the second photo and wax poetic about the virtues of inert substrates but that would be ridiculous because it would be ignoring all the other factors that contribute to plant health.
Cheers,