This plays into the nature of the story of the drop, trying to take down a 'big bad.' The cards featured in the drop are all fantastic, and even the lowest-value card, Lightning Bolt, is a staple in every format it's legal in
Like the other enduring cards, Enduring Courage returns as an enchantment after being destroyed, which this card is hardly phased by compared to other Glimmers, only being a 3/3 creature otherwise
This turns Equinox from a risky inclusion in the 99 into an auto include, as it becomes a repeatable free counterspell
That's a powerful way to ramp up your mana base with some mana-producing artifacts, or you could just have two Darksteel Colossuses bouncing around
Sure, you have more cards to choose from if you go with more colours, but you also complicate your mana base