My issue with making it a primary impulse engine would be that it would seem to not be along the center of mass...
Well, impulse engines never
are. Unless one fine-tunes the mass of the nacelles, the one element that can be assumed to noticeably differ in density from the rest of the ship. But even that only saves one ship class at a time; what works for the Constitution
refit won't work for the Miranda
So far, we've seen that the glowing bits of impulse engines can be of any size, point in any direction, or even be completely absent (TOS!), so we can probably dismiss their role as a rocket engine of some conventional sort. Perhaps their glow is harnessed for boosting propulsion by 0.7% in some designs, while in others the as such meaningless boost is not incorporated (the way some aircraft piston engines derive extra boost from venting their exhausts while most don't).