When colosseum event wast first started - it had brilliant concept to involve weaker girl cards into something meaningful, another good idea was freedom to choose different girls/deck vs different opponents/bots and utilize the whole roster.
NOW, anything below Legendary is redundant/ barely useful - these girls are free spectators to watch the colosseum fight between Mythics with chance to get cheerleading when your deck has empty spot for them (btw this is sarcasm/pun - and it is intended)