It isnt easily undone through Force of Will or Force of Negation, which are typically the go-to defensive options when a deck needs to tap out to cast something
A 4/4 with trample for three mana is already fine, but the recursion this card offers is whats really appealing, as if your opponent trades a creature off in combat with your Troll, the Aura it leaves behind will help you ramp out even more threats, emptying your hand more quickly to overwhelm your opponent
The thing that is currently holding back Valorous Stance is that there is only one deck in which White is playable, and that's Monowhite Aggro, which is too fast to be all that interested in the card when they have options like Brutal Cathar and Portable Hole
Cube has such a motley assortment of cards that you never know what you are going to have to deal with
people wanted them, and would do anything to get a hold of them