I throw my 22 cents worth in...
The quest for purity is interesting. If we base it only on logic, the conclusion can be flawed - sometimes it requires a greater perspective of what purity actually consists of.
If you look at a concert hall, in fact it is in effect something that is man made. Something "truly" pure - you should record concerts in a cave, or in a forest for example, or some kind of a natural amphitheatre shaped rock, strictly speaking. In fact purity or aesthetic perfection is something itself that is determined by our own innate sensibility, it is a. passed on generationally by other people, b. accumulated ideals, refined over time, c. intuitive d. perceptive - experiential, for it to be felt as an artform by other humans. Aesthetic's are ultimately - by the balanced individual (which there are rarely any if any [[:)]] ) harmonic with natural principles.
To abstract it slightly more, a concert hall itself is basically an algorithm, the type of bricks, mortar, paint, size, shape, contents, position of the instruments, and position of the microphones, or ears, if you are listening live. A concert or musical space is typically "designed" algorithm using physical parts that are designed to give the most aesthetically appealing sonic imagery for a wide variety of content (or specific type of content as the case may be - piano, orchestra or a rock band). This type of setting is made by people and is typically modelled on controlling or harnessing certain natural principles of reverberation to achieve the best results. Even if it is modelled on sound itself, not just space, then it is still an algorithm in effect. You take a + b + c and get your result. It is a reverb engine if you want to call it that albeit inherently complex more complex than any one computer or rather software today can calculate.
A synthetic reverb itself is also essentially an algorithm running inside a processor, with a set of parameters. Like a concert hall, it is designed by people, for the purpose of providing aesthetically pleasing sonic space for a variety of content. The difference is that it is virtual and models itself on natural principles of reverberation to achieve the best results - the mediator for this is the engineers ears and aesthetic sensibilities attached to those ears [[:)]]
If you then take an IR of either one of these scenarios, either one will be inherently "flawed" compared to the original, being basically a recording - a second generation from the original, but relative to the price of a. building or recording in a concert hall, or b. buying the necessary hardware, that is up to the user what is best for their budget.
I have bought Peter's IR sets and they are very good I use them in altiverb although I must confess I haven't had any chance to use them much lately beyond a quick try out - so I will give a more full review at another time when I've used them a lot more.
What is interesting is that for a concert hall to sound "right" in a recording it depens largely on the microphones used and also the placement of those microphones, and so sometimes, for the sample based orchestra, it can be "easier" to get an aesthetically pleasing sounding from a synthetic IR which is designed on sound and often, gets the overall sound placement effect on the instruments aesthetically "better" than a real world IR, which, true to it's real world counter part can be difficult to "tame".
In effect there is no conclusion to this post, I like many others are really just looking for the best and most aesthetically beautiful sound we can make with the limited budget we have (limited = relative) and the ability to buy a whole set of reverbs from the TC600 and 960L for a very small fraction of the original machines, and they are very useable (as I said a more thorough review at another time from me) if not the real thing, it's worth it for this composer for one.
Miklos.