The thing is, after a rune is 'prepared,' as swiftly as it might be or might not be... using it later in the heat of battle should count as a matured prep because it would force Myron to concentrate on only that ability. He wouldn't be able to prep any other spells whilst making sure that rune is doing the best job it could, and it would likely consume an entire action for him, even though the rune was dormant until then. So essentially it would still follow the standard usage form for preps under T1E. Since there's already restrictions to Myron's cast speed, I see this as a way to ensure the end result isn't a waste of extremely important time. The con is that even his faster spells aren't inherently fast at all... the pro is that he has more options at his disposal if he prepares the right rune combinations.