Abilities that require you to tap the permanent theyre on certainly feel like they can only be used once per turn, but you can use the ability again if another ability untaps it
Not only does it provide an untap just for attacking, it gives a +2/+2 buff, and Totem Armor, whichll save your fattie from a board wipe
The addition of Flash and Flying makes this card an absolute powerhouse for tempo-based disruption
I think even a single target Replenish would put serious constraints on the format and the types of designs WotC could safely make
Dualcaster Mage is great in high-powered decks because it goes infinite easily, but Reverberate, Flare of Duplication, and Increasing Vengeance all work
While Im not sure if it truly makes the cut in cEDH, you could also cast Return the Favor to make your own copy of the enters-the-battlefield ability of Thassas Oracle to win on top of their ability on the stack