it's more a choice than THE ultimate solution.
sennheiser always believed that one driver was the best solution because it was better to work on frequency response than to try solve phase and crossover problems.
obviously in multi BA drivers the other choice was made, for easier work on frequencies and to get a better range than just one BA (but still usually less than one dynamic driver).
the purpose of several bores as you can guess is to keep each range of frequency separated until the very last moment to avoid possible interference. usually the trebles suffer the most in multi BA IEMs(that's why Suyama san tried the metal tube for the treble driver on the 334).
my take on all this is:
+ you get a 2, 3, or 5ways almost up to your brain so each driver can output a clean sound with reduced interference from the others
+ each bore being physically at a different place creates an artificial positioning that will help telling instruments apart in space.
+ it also helps(with the FR) to create an original headstage making the IEM kind of unique (for better or worse).
+ some trebles might end up having a slower roll off.
+ the tubes when using several bores might be used to control the phase between drivers(longer or shorter tubes), it might not be needed, and there are other ways, but it looks like a simple way to do it if needed.
- each crossover will either have 2drivers overlapping the same frequencies, or there will be a hole in the FR graph. it's pretty much impossible to hope for perfect filters that will make one driver stop exactly when the other one starts. (but that is a problem in all multi drivers, not just those with multi bores.
- some instruments will make sounds that will need 2 or 3 drivers to be transmitted, if there is a very noticeable spacial positioning effect due to the bores, it might seem unnatural.
- same with sounds going up or down in frequency(let's say guitar of piano playing some notes at 200hz then steadily going up to 2khz), it might seem as if the instrument is moving or jumping. can be fun, or disturbing. and for spacial sound effects (someone walking, a plane passing... that kind of stuff) if you pay attention, you might notice the plane jumping from a place to another at one point in the doppler effect. so not realistic. with several crossover and a single bore, chances are that the spacial cues will work better because the sounds will mostly come from the same area. you might still be able to notice a small jump from one point to another but obviously one bore per driver adds in panning precision, again for better or worse.
I usually find that tremendously fun, to me headphones don't create a realistic soundstage anyway(180degree intead of 60 for speakers or even less for a real band). but someone looking for the "real perfect sound and imaging of headphones" should simply avoid multi drivers.
if you like multi BAs then multi bores always sounded better/funnier to me(didn't try 5bores, 4crossovers seem a bit overkill to me for what is actually needed, but maybe it's the ultimate fun...).