While the cards Master Ball Mirror print in and of itself adds exclusivity, this factor has an additional aesthetic factor pertaining to Gengar
Its a wonderful little self-contained story about someone sacrificing themselves so that they can bring allies back from the afterlife with them
Both markets have examples of high-value cards, and both can serve as good long-term holds if you choose wisely
3 O'aka, Traveling Merchant Finality Counters Don't Have To Be So Final Like Hermes, you're not going to be running O'aka as the commander
The main question is, do you just go for value with this green land, or go for the full combo-turn and make it a copy of something silly like Omniscience
More importantly, you can sacrifice the land to bring back a Bat, Lizard, Rat, or Squirrel card from your graveyard to your hand, which is a solid utility land in Standard and Commander