First, you can substitute it for any one Fusion Material Monster for a Fusion Summon, as long as all other Materials are correct
Today, it holds up incredible value, and Glaceon is arguably one of the more naturally beautiful of the Eeveelutions something which lends itself to this cards value
Brawl Legal Vintage Legal Timeless Legal Commander Legal Pauper Not Legal Oathbreaker Legal Penny Not Legal Toolbox Notes and Rules Information for Vanille, Cheerful l'Cie: Each face of a nonmodal double-faced card has its own set of characteristics: name, types, subtypes, abilities, and so on
As for ST21-014-monkeydluffy, the rested Don he attached to a Leader or a Characters bumps your aggressive playstyle, especially if you rely on Don to activate certain effects