Should you keep playing? Well, you didn't win the game. Lol.
Costs no energy so it's just extra practice the way I see it. I'm sitting on 500k right now. Anything I get from them is a bonus and it only takes 10 minutes to run through Epic. Just go as far as possible and use no items. When all my top champs are being used between AQ and AW I usually fall just short. Get to the final fight and quit once KO.