I think that the number of BA's in an IEM is not a strict representation of its audio quality. The crossover implementation and tuning of the drivers is much more important. Take for example the Westone 4 and EarSonics SM3. I actually preferred the EarSonics SM3 much more than I did the Westone 4, even though the Westone 4 had more BA's than the SM3. Perhaps that is strictly my signature taste, but I also felt that the SM3 was almost technically as good as the Westone 4, but fell short. And I believe it fell short technically because the Westone 4 sounded more effortless throughout the frequency range--and I give credit to the additional drivers that the Westone 4 has over the SM3 for this technical superiority.
I believe that by adding more BA's, you can definitely reduce distortion and create a lot of headroom for spikes in dB for live performers. This is what Jerry Harvey mentioned as a prime reason for the 8 BA's on the JH16. I also believe that accuracy can be increased with more BA's, as there is less stress on each driver to produce a certain frequency range.
But overall, after having heard so many dual/triple/quad driver IEMs, I can say from my observations that more drivers generally means a better sounding IEM (with exceptions). Again, the crossover implementation and tuning are crucial. Having heard the Westone 3, Westone 4, Shure SE535, EarSonics SM3, JH16, JH13, and other multi-driver IEMs, I can safely say that to my ears, more BA's offer a cleaner, more effortless sound.
Out of all the multi-driver IEMs I have heard, the JH16 sounded the most effortless to me, followed by the JH13 and the Westone 4. As you can see, these are correlated with higher driver counts (that of course does not mean such a law exists, it is just my observation). Also, I believe that the complexity of implementing a good crossover and a coherent sound becomes difficult with more than 3 drivers. There is a lot of phase correction to be done at that point, with so many drivers it is hard to create a single coherent sound. Additionally, from my experience instrument separation becomes discernibly better with higher driver counts.
Overall, I believe higher driver counts can definitely be a beneficial thing for IEMs if it is implemented with care. The best improvement I have heard with higher driver counts is a more realistic sounding, organic representation of an atmosphere (jazz club, orchestral setting, etc.). The ambiance is captured better with more BA's in my opinion (perhaps this ties into the distortion and accuracy part, less work per BA). And finally, I feel like the music sounds more effortless with more BA's, but this of course can be true with fewer drivers as well.
I think it isn't an exact science. A lot depends on how the drivers are implemented and how the engineer wants the IEM to sound. It is more of an art than a science (and then there are personal preferences, which change a lot of perceptions as well).
Edited by SolidVictory - 9/10/11 at 6:28pm