But, structural integrity failure is the wing coming off, or something equally disastrous. You don't lose the wing on a Fighter when the armor's gone. It starts taking internal damage which weakens its structural integrity - hence it has internal structure. It just happens to be so open that there's no IS tracking. I do believe that when the SI is damaged, the craft has to make an integrity check or fall apart. It's been a while, so I'll have to double check.
You've seen what vehicle frames look like, right? How much of that actually covers the whole vehicle? Usually its just a frame. The way BT armor is applied, it's actually hard to imagine the armor being the hull, like we get with current MBTs. It might be the case that they are, but I'm not seeing it with the repair rules.
So, with that in mind, tanks shouldn't have an internal structure where we can track individual points per side. To cave in a wall, you have to break the structure, right? You can ablate the protective coating (armor) all you want, but as long as the frame is generally untouched, the structure holds. Vehicles, while not as big as some buildings, are still big boxes. Even the smallest takes up as much volume as all three torsos and head of a (light) BattleMech. (Aside: You can't trust the minis. The Savana Master is effectively a mobile Mech head, and the Gabriel is stated to have a crew of more than one, making that mini too small for what it should be.) So, that's a lot of space for that kind of internal structure. I imagine there's a lot of open space between the ribs or frame.
Now, you could make an argument for the hyper accuracy of BattleTech targeting systems, which is an argument I'm fond of, and suggest that attacks can easily pick out the support structure with relative ease, thus necessitating the need for IS values. But, then we need those same values for aerospace, as well.
And, if it turns out the armor is integrated with the structure, like on modern MBTs, then you have an issue, because the two values should be indistinguishable. But, even if the armor is full of holes, it can still retain its form until a significant impactor wrecks the form, like a physical attack. In that instance, don't you think there should be some sort of structural integrity check?
I do.
The idea of proposing that Vehicles use structural integrity started as a form of simplification of vehicle sheets by getting rid of superfluous internal structure values for each side, while still allowing for something beyond the armor. In a lot of ways, it makes sense to apply, for consistency, to anything not a Mech. When a Mech kicks a tank, or it's rammed, it makes sense that it should make a 'cave-in' check (structural integrity). In fact, I'd highly recommend that buildings should be subject to the same kind of check as well whenever they take any damage whatsoever.
Sure, in some ways, it adds an extra step, but it feels natural, and is easy to envision.
The other direction, while simple, is inconsistently applied. If tanks have IS, then so should (Fighter) Planes, DropShips, and even Warships.
Aside: In fact, I've taken on a personal project to map out the stock three DropShips so that when they take damage, especially grounded, they can lose whole sections like a Mech, and only lose partial functionality, unless it's something serious, like the center torso (main drive/keel) or head (cockpit/bridge). It's especially for when a DS is landed, since I envision ground units being able to pick out targets on such a large structure with relative ease in spite of ambient ECM. The ship can't move while on the ground, so that armor can't protect the entire side that easily. Just look at the limitations of Mechs and tanks when it comes to armoring?
I hope you see where I'm coming from on this before you go making yet another argument for the need for Internal Structure Values.