Actually, there are a few reasons. First, lack of aux sends, second, lack of tape returns, third, bad pots. The system 8 consoles really do sound pretty decent, but they didn't have the same preamp that went into the Saber which is a MUCH better Allen Heath console. They definately sound better though than something like
a GL2. However,
the GS3000 preamps are just as nice, but in differnt ways. The System 8 will never give you that smooth clear high end. Its always a little dull. What was nice about the system 8 is that the low end had some nice thick warmth to it. The GS3000 (which is a different pre than the first GS3's) doesn't quite have that low end richness of the system 8, but the highs are much more detailed and airy. Not quite like a nice outboard pre, but much more so than the system 8.
Also, the system 8's really aren't that rare. In the last few months I have seen quite a few of them for sale. In fact, I may even sell my old one soon (just sitting on a shelf). Personally, the system 8 consoles are certainly not a bad buy. In my opinion though, the GS3000 is worth about three times as much. If nothing else just do to the fact that it's still servicable by Allen Heath, is fully in-line, has more EQ, more auxes, more meters, midi functionality etc... etc...
Either one is going to sound much better than a Mackie or Yamaha or Behringer or little Soundcraft.