Hey there,
Altiverb (AV) certainly gives the user many options, such as speaker placement, separate control over reflections, EQ, and all other kinds of fiddly things but I'm not really sure how much of this is useful for realistic mixing, and how much of it is actually getting in the way. These days I tend to use a rather minimal amount of features in AV and I'm getting better .. or rather less frustrated .. results.
The main thing that altiverb has going for it is a huge library of Impulse responses (IR's) .. however other reverbs are catching up with this. VSL have their room packs for the VI reverb and for MIR which offer many new nice room sounds. AV is an awesome reverb for sound engineers who are seeking very specific room sounds like 'a small bathroom', or 'cupboard', IR's .. but in my opinion it is not the only reverb that will offer acceptable sound for virtual orchestra.
Space designer (SD) in Logic will actually offer the same kind of quality as AV. The only thing to get right is to choose the right kind of IR for SD, and there are many on the net that you can pick up for free. all one really has to do is just use the reverb correctly .. meaning that if you assign your instruments to your reverb with a bus send / return configuration, then you will be able to get as much benefit from this as you would with AV. Perhaps the biggest difference is that you won't be likely to get the 'TODD AO' or 'Paramount stage M' IR's for Space designer .. but that doesn't mean that you won't get a good IR out of SD if you look for it.
I'd say it's worth while investing in AV if you have the money as it will give you more choices, and that's never a bad thing. But if your budget is tight and you already have SD then I would say that the SD reverb should suffice and be able to give you a great sound. Beat Kaufmann warned me early on when I asked him about AV that I shouldn't expect it to solve my problems .. and he was totally right. AV seemed like a great choice to begin with .. but I've noticed as time wore on that it was really more bother that it should have been. And If I'd have stuck with SD I would be 500 Euro better off :P
As for achieving homogeny amongst your different libraries it's difficult to say how this is best done. I use a limited amount of EWQL instruments in my recordings (usually percussive instruments) and I find that using the middle range mic positions and assigning a small amount of reverb to them (the same reverb that it going to the VSL instruments) they tend to sound fine. But if you're using lots of different libraries then yes I can imagine there being depth issues. This is one reason why I prefer to use one library primarily. Again if you use reverb in the Bus send / return config then that gives you a lot of control to be able to use less reverb on the libraries that have more 'native' reverb already included.
All in all I'd say hold out for MIR and for the mean time just try to get the best out of SD. SD is quite capable of giving you a lovely sound .. as with all things it's just a case of learning to manage it and trying to find the best way to use it. MIR will most certainly give you the exact level of control you could possibly ask for .. with much less fuss.
As for the issue of whether or not to use one reverb or to use it in sections. I've tried both ways and one offers more possible control than the other .. but I've found that really both ways will give the same sound ultimately. In Pro-tools 8 I have the sections separated out into 4 groups, mainly so I can avoid overloading the buses and have control over the volumes for the sections .. but often I find that this approach really doesn't help improve the depth or sound of the reverb. In logic I've used just one bus reverb (and one on the main output), with the right kind of attention to the bus sends this really gives me all the control I need to adjust depth. Really I think it's all down to what IR you're using and how much closeness or depth you're using on your instrument.