However, in Modern, with the Tron lands able to produce tons of colorless mana quickly, this could be an option. There are plenty of Burn decks in Modern. So, this can shut down anything that targets you, including discard spells like Thoughtseize and Inquisition of Kozilek. However, despite the fact that it might fit in the strategy, Modern Tron decks have rarely used this card.
The other plus to Orbs of Warding is that it reduces not only the damage of creatures dealing combat damage, but damage from their abilities, as well. Eidolon of the Great Revel is a great example of a card that the Orbs protect against by cutting its effect damage in half. This card also makes Lingering Souls pretty bad unless the 1/1 flyers become equipped.
(Although it no longer matters in Modern, Orbs of Warding also shuts down the Deceiver Exarch/Splinter Twin combo by making those infinite Exarchs deal 0 damage to you. This forces your opponent to use Pestermite, a much more fragile card, or Bounding Krasis, instead.)
Witchbane Orb has seen play in Legacy sideboards before. While this is one more mana, it will eventually see play when the metagame demands it. That may even be in Legacy. For the most part, Orbs of Warding has just been a nice little tool in some EDH decks. It saw only fringe play during its time in Standard. But, a day will come that Modern sideboards will be more than happy to include a copy or two of Orbs of Warding.
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