You just said NO tweeter does VERY well off axis, that's an absolute statement if I ever heard one. While there is on axis and off axis in terms of angles, the response in a car is very different than what you get in a home. The early reflections in a vehicle are the majority of the sound you hear, JBL actually did a study on it when building their MS-8. Taming the reflections and making their response as linear as possible should be the goal if you want what you hear to sound correctly. In a home that's basically always on axis, in a car it just doesn't always work. If placing a driver on axis also creates early reflections that have a response that doesn't match that of the driver you end up with bad polar response and a very small sweet spot as a result. IME on axis generally preferred, but not as often as you seem to be indicating. I'd say in 3/4 of my installs tweets end up within 30 degrees of on axis, but the angle of each tweet varies quite a bit and I never know until it's done.
To say that on axis is the only way to go is pretty dogmatic and misleading at best. Cars dont' break the laws of physics, but they do require us to look at other uses for the laws as the environments and the effects of the laws that govern them are different than in a larger room. Reflections dominate and that makes the way you have to look at things change. I've never seen anyone in a home put their midbasses at their foot level, however in a car it works very well. It's a trade-off from "ideal", but it's more ideal than what's ideal in a home, which would be near head level in most cases, certainly not floor mounted. In a car we don't have even PLD's and that becomes the bigger issue and we take advantage of the lack of beaming in the midbass region of a smaller driver. In a home, that's just not necessary to have to deal with. Same deal here, we have reflections so dealing with those is the BIGGER issue than on axis vs off axis mounting, they are both simply a means to an end. In a home to get to the correct result you place them on axis in a car you place drivers where the direct to reflected sound gives you the best polar response between the two while generally trying to keep even PLD's.
If you have a tweeter with the correct dispersion characteristics, (usually mounted to a waveguide) then off axis mounting could easily be ideal. A little beaming near the crossover point can even combat side bias for both seats at the same time, which is something that cannot be done electronically.
