In fact, this flashback spell is the only one to see play in Magic's oldest and most powerful formats: Legacy and Vintage
From there, you can pair these flicker effects with cards like Peregrine Drake, Palinchron, or a basic land (in the case of Ghostly Flicker) for infinite mana, Overwhelmed Apprentice to mill your opponents out, or Venser, Shaper Savant to return all opposing permanents to their owners hands, just to name a few of the possibilities
I know 21 commander damage got decided on due to the original cycle of Elder Dragons having 7 power and you getting hit three times equaled 21 but the book quote is just so fitting for our format
For only the cost of five, you can play Mad Hatter and use him to quest for three lore
For example, if the spell has a kicker ability and was kicked, the copies will also be kicked
Pauper also applies for new commons