I hope defender never rotates out of being evergreen
For example, if you cast Junkblade Bruiser (a creature with a mana value of 5 and an ability that triggers whenever you expend 4) by paying its full cost of five mana, you'll already have spent four mana on spells before Junkblade Bruiser enters, and therefore its ability that triggers whenever you expend 4 won't be able to trigger at all that turn.
Either way, you get paid off
Enduring Curiosity Gameplay The engine behind Dimir Midrange, the formats second-best deck, this enchantment from Duskmourn provides a relentless card advantage
White mana allows you to play virtuous cards such as Serra Angel, a flying creature with offensive and defensive ratings of four each, which makes Serra Angel a strong card but by no means the strongest in the game
An extra mana and an extra card can make a huge difference in a game and Time Walk makes that happen