While he does cost a steep 6 mana to cast, he also comes with an ETB tutor trigger, and if it's Hammer of Nazahn he even saves you the mana to cast it
These packs are NOT FOR SALE separately anywhere (stores could get sanctioning taken away if they sold them, see the Store Handbook for more information), and the Pokmon Leagues decided themselves how they were given out (League attendance, extra tournament prizing etc)
Try to draw all your necessary cards while investing Energy into Dialga ex the first two turns
Id run at least five board-wide protection spells in those go-wide decks, and more if I could swing it
It can also function in a way that leaves certain permanents in play (like Smothering Tithe, or a hatebear) while saving your tokens
If you have ways to mill your opponents, Memory Plunder gets better and better, and in general, I feel like most Dimir decks should be finding room for this spell