It has to do with samples, rather than live performers. If you take 8 live players and record them together of course it will sound lke an ensemble. But if you take 8 solo sampled players and layer them together it will not sound like an ensemble, but will have disturbing phasing effects and a general artificial quality. This is due to the extremely complex acoustics of string instruments. You may notice that this is NOT true of woodwinds. In other words, you can create a very good woodwind ensemble out of solo samples. But with strings it is not just an option, but essential to use the Dimension recording technique. With brass it also helps, but is not quite as essential. For example if you used the solo C tumpet and Bflat trumpet along with cornet, you can create a very good sounding ensemble, especially with MIR providing the reverb and space. Though the Dimension Brass add a very easy to use function of the same trumpets, horns or trombones playing in any combination which makes them good for general orchestral use.
One other thing I've noticed on Dimension Strings is that though they are not designed for solo use, the blending works just as well for 2 or 4 players as for all 8 players.