You are severely miscalculating the potential and effect of Mysterious. The push effect does so much more than Shadow's ability in most cases, for one. Shadow's ability can only affect a single AP from a single model, if it even succeeds, whereas pushing in reaction to a charge, or even the first attack of an already engaged model cancels at least one, if not more of the enemy's APs. If the leader is engaged by multiple models from the previous turn, and especially if said leader has a melee range of 3, pushing it might result in the effective loss of 3+ enemy APs, if you're then out of their range, but they're still in yours. It breaks charges, flurries, second attack triggers and can cause the loss of melee expert AP, among other things. Outside of simply cancelling enemy AP in combat, the condition can be used simply to add 3" to the leader's movement, to enable a charge or put them in range, or it can be used to slide out of sight or into cover you couldn't otherwise reach, again potentially cancelling multiple enemy AP. If you can't think of how 3" of reactive movement can be used to great effect, you're really not thinking hard enough. I certainly wouldn't trade our effigy for any other.