Play it turn 1, swing in on turn 2, then put Muxus, Goblin Grandee into play and get another five goblins to boot
Probably not unless ninjas come back, unfortunately
With other complementary artifact mana sources like Thran Dynamo and Grim Monolith, decks could count on being able to get the kind of mana you'd need to justify packing the cards that would let you use the mana from Metalworker to deliver the knockout blow, and their presence lets you turn that Metalworker mana into permanent giant amounts of mana rather than just a one shot deal
For example, 3 Rude Kaisers may result in an Anti-Magic Fragrance, Blue-Eyes White Dragon, Gorgons Eye, etc
Since Explorer is slightly lower-powered than Pioneer at present, even corner case cards have the potential for major impact
What is the rarest type of Pokmon card