They're worth ranking up as long as you decide they are. If you get someone OP like Onslaught, Serpent, Enchantress, etc, it would be crazy to go "Wow, this character is really strong! Unfortunately, it's a 5* so that means I can't use them."If a 5* Champ can fill an immediate gap on your team, absolutely rank them up. Especially because 5* mats are a dime a dozen nowadays, even for lower progression levels. You won't be hurting yourself in the long run.