Void Mirror is an unassuming little artifact that actually threatens to a potent sideboard card in Modern.
So what exactly are we looking to shut down with Void Mirror?
While it's true that Eldrazi and Tron decks can and often do run colored mana (and may have to adjust their manabases due to the existence of this card), Void Mirror can often make it extremely awkward for them to cast their big spells. Any deck relying on cascade or other ways to cheat out cards with no mana cost (such as As Foretold or Electrodominance) are hit even harder, as they simply have no way to resolve these spells.
Force of Negation and Mishra's Bauble (or anything else that costs 0) are a few more examples of commonly played cards that are hurt by Void Mirror. While these on their own might not be quite enough to justify the inclusion of a 2-cost artifact in your deck, it's definitely a card to keep in mind for certain metagames.