Even if you throw out balance, and toss away the fact that any crossover ships would be so rare in the overall universe that it's not really worth the effort to create them, it would destroy the thematic faction element completely.
S&V gets away with a couple crossovers because they take them in an entirely new direction. The new upgrades make the ships play significantly different than their counterparts. They feel like S&V ships.
Throwing a TIE into the rebel lineup wouldn't have that effect. The examples being slung about are all stealth mission, so they would be the exact same ship, just with a different pilot. So it would just be crossing lines for no other reason than a few people want it. It would add nothing, while detracting plenty.