It would make plane combat more realistic, it'll be super important for boats (for obvious reasons), and if done in an even semi-realistic manner, could provide MTS with another opportunity to stand out from other vehicle mods.

The general consensus around fuselage damage from a mechanics standpoint seems to be as follows:

• It should be based around a groups system that links different sections of the model to hitboxes (for instance, a model part with a name ending with g1 would be linked to the first group of hitboxes)
• Damage to a group would have a chance of different results defined in the json. If a group holds fuel in an non-self-sealing fuel tank, each shot would have a chance of creating a fuel leak. If the group takes fire damage with a fuel leak, there's a chance of it igniting. If the group contains control surfaces, there's a chance the surfaces will stop responding. If it contains lights, there's a chance the lights will fail.
• If a group is damaged enough, it will vanish, and, depending on how it is defined in the json, the plane will lose certain capabilities. If there are parts dependent on that group, they will fall off. If the group is defined as a wing, it will create lift imbalance based on the size of the group relative to the remaining wing groups.
• Probably a way for groups could be dependent on other groups? So if you had tail fin groups and a tail body group, if the tail body gets shot off, the tails die too.

However, the visual part might be trickier. The two main ways I see are to have MTS generate damage states on its own, or to have the modeler pre-define them. MTS could randomly blacken or erase pixels in groups, for fire and physical damage respectively, or the modeler could create "damage state" textures, like what War Thunder uses, that MTS switches to or overlays in response to damage. The second option could avoid some weirdness inherent to the first, like internal damage not aligning to external damage, but would be more work for content creators.

I know this will be a PITA, but it also strikes me as fairly important in the long run, both for proper boat behavior in response to damage and for MTS to be one of the first vehicle mods with a relatively comprehensive and accurate damage model.

still doesn't deal with triplanes

Also, self-sealing fuel tanks do leak for a bit while they're sealing.

It does deal with triplanes, actually. "If the group is defined as a wing, it will create lift imbalance based on the size of the group relative to the remaining wing groups."

This would require a complete overhaul of not only the collision system (to segment collision points), but the core physics system (to handle asymmetric lift), the rendering system (to handle parts of the model not rendering), all OCP vehicles (none of the planes are modeled to support fuselage loss), and the parsing system (servers don't know what models are like because they can't see assets). Not to mention it would require finding a way to parse models and determine where to place damage indicators, as well as SIGNIFICANT increase in JSON for packs, which is already a high barrier to most modelers. No, this system of combat is far too complex for MTS, not only from a technical standpoint, but from a "Don literally doesn't have time to code this and features and force people to overhaul their packs for a weapons mod change" standpoint.

TL-DR: Too complex.