Whew ... that's food for a book rather than a forum message ... 8-)
In principle, you're right: A typical orchestra should be located in one room at any time (if you're aiming for absolute realism, that is ... most pop- or rock mixes _live_ from breaking this paradigm - but that's a different topic).
But "one room" doesn't necessarily mean "one reverb" (in our understanding of the word) - quite on the contrary! In fact one single room will sound totally different depending on the sound source itself, its radiation pattern, its pitch, its position, and of course depending on the listener's position, and (in case of a recording) of the polarity pattern of the microphone(s). IOW - yes, every source will create its own unique reflection- and reverb-pattern. Using just one single reverb signature for all instruments of the whole orchestra is not unlike sampling the middle C of a Grand Piano in one velocity, stretching it across 88 keys and trying to play Chopin etudes with it. 8-)
As a matter of fact all of these thoughts (and some more) led us to the development of MIR - our Multi Impulse Response reverbation and mixing engine. Here, the reverb "belongs to the source" _and_ the listening position at the same time, just like in real life. Of course, phase cancellation and other interactions between the resulting individual reverb trails _will_ happen, but that's part of the game, as similar phenomenons occur in a real hall, too. Actually, these interactions are one of the reasons that the blend between the single instruments happens at all, which allows us to achieve that typical "orchestral" sound in the virtual world.
... that said, there has always been a blurry line between "pure realism" and "perceived realism": Many great orchestal performances from the best halls on this planet have been enhanced on behalf of additional reverb, delayed microphones, even modulation and so on ... so in the end, we come back to the archaic First Law of Sound-Engineering: "If it sounds right, it IS right". There is no "best" solution, just several valid ones. The "one reverb" approach has worked for you for so many years - do you feel you're missing anything nowadays ...? Or did it just work due to technical and/or financial restraints ...? Food for thought ... :-)
Kind regards,
/Dietz - Vienna Symphonic Library