This would give everyone the chance (if they play well enough) to add an extra cover or two to the required characters before the Shield Training event rolls around.
Given Quake and Miles are both older characters, it would help those of us who have them rostered but not yet fully covered. (I’ve championed Quake but my Miles is still at 2/2/3.)
It may also drive HP sales, because if someone doesn’t have them rostered but they pick up their cover as a progression reward then they see that character is required for Shield Training, they may consider buying some HP so they can roster them.