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You can do both, for example Hearthstone had this exact mode, the reward was small but it was enough to do each week even though I didn't really care about the mode. Giving incentives to play is never a bad thing, the problem is when the rewards and incentives are not aligned. For example if competitive players were forced to grind it each week that would suck, but if there was a small reward for winning a few games or something it could be a nice break.