When I think of "unique" and "rare" T206 cards, it's the scraps/freaks/miscuts/ double&triple and multi back print, etc. That's why they are and probably will continue to spike in price. And, if the prices weren't so high, way more collectors would be going after them.
If you can't afford a Magie/Wagner/Plank/Doyle Nat'l, and want rarity in your collection-this is where you end up. Or, go for tough front/back combinations.
The supply is limited, the cards are unique and rare, and I still don't think they have topped out yet.
As far as other sets, I would surely look at the freaks/miscuts/scraps/etc. the exact same way, if I collected those sets. The E92 shown earlier in the thread would be way more desireable(to me) than the regular one with no flaws, if I collected that set. That's just my take on it.
Sincerely, Clayton