I wonder if this makes an endless turn possible, when Graverobber/Lurker is in the Kingdom.
Won't you eventually run out of Necromancers to play?
Ferry Graverobber, then trash a Graverobber and a Lurker. Now Band of Misfits can do everything that Necromancer can do, and vice versa. Do you think BoM enables endless turns? If so, how?
Note that Necromancer
2nd-order trashes removes from the game exiles turns the played cards face down, making them no longer Necromancable.
I came up with an Ambassador/Villa/Bonfire loop; the full combo is 9 cards, several of which cost less than BoM. The 10th card in the kingdom could be BoM, and it could imitate some of my combo components, so I guess in that sense BoM
does enable endless turns. However, as a general rule the number of cards not in play tends to go down during your turn, to a lower limit of 0, and BoM doesn't violate that general rule. (Bonfire and Mandarin do.) Necromancer also doesn't violate this general rule, so I don't see why it should help you go infinite any better than, say, BoM (or Village or Smithy).
There's a second general rule that cards in the trash stay in the trash (and thus can't be played). Trashed-face-down is just a second trash pile which satisfies this rule, so I would guess Necromancer could at most double a finite effect, not make it go infinite. Note that Rogue, Graverobber and Lurker not only violate this rule, they put the de-trashed cards back in your deck (uh, discard) which Necromancer doesn't do. This makes me skeptical about the other way Necromancer maybe has loop potential.