Plus, you get to play cards without paying their mana cost - omniscience style (once per turn, mind you)
I could also see this as a sideboard option in a similar role that Otter Ral plays in Izzet decks
While she's a popular legendary card on her own, the Enchanted version gives her the villain look we all remember, with blazing eyes and a menacing demeanor while perched
While other players can stop you drawing from Esper Sentinel, more often than not, they won't want to wait an extra turn to play their spells, more or less ensuring the card at least replaces itself
Deadly Rollick might not be the best of the Commander free spells (that would be Fierce Guardianship), but it still feels devastating when you cut off somebodys combo or remove their best creature with no mana
Most Expensive Magic Oath of the Gatewatch card list & price guide