User blog comment:Linathan/Squad Spotlight: Fun Being F2P Part 1/@comment-26464145-20151019072423/@comment-24570336-20151019101344

It's a guideline for a F2P squad, where people don't have enough gems to get what they need. If a player gets a Darvenshel then that's fine and Barienna can be replaced if that player chooses to. This is just to address that you don't need Rare Summon units to beat the game. Therefore, it's a form of motivation.