Just because the driver is “capable” of it, doesn’t mean it’s ideal. There’s many reasons they might choose not to bump it up 6 or more dB. The consequences on the mids/highs get messy, the cost of components to compensate in the crossover can go up or become impractically large physically, the thd consequences can be substantial, the phasing related consequences of aggressive analogue filtering can cause audible artifacts, the list goes on.
Companies aren’t stupid, there’s a lot that goes into designing iems. Source: it’s a large part of my job. (Not at moondrop, unaffiliated)
I wouldn’t assume theres always reason features are present, especially in audio where marketing hype and snake oil run so rampant. Like dipping all your screws in liquid nitrogen before assembling your speakers… lol. I’d say right now the biggest reason is just the hype train, that’s what’s in right now. From a technical perspective you get a little better power handling/output level without increasing thd. That’s essentially all, aside from it just being geometrically convenient to put them close together. As far as effects other than bass, there really aren’t any. One could argue that it makes the iem slightly more efficient because by acoustically increasing the output of the bass driver you don’t have to attenuate the high frequencies as much to get a bass shelf, but that’s not a huge improvement.
They could be leaving some headroom for the dusk as you mentioned, but I would argue there’s nothing meaningful outside of thd that a second dynamic would give you that you couldn’t pull off with a single. If that’s the case then that would be a small improvement to thd, but potentially a meaningful one.