my group is rank 11 currently and we have 13 spins per day. i'm still able to roll 13 times and gathering some (very few to be honest) currency for leveling up the group skills.

but i do it because in average out of 13 tries i get 2-3 times coupons. sometimes happen to get 0 times coupons but the following day happens to get them 5-6 times.

it's worth it when the group skills are maxed and when you don't find any real usefulness for coins (so after level 90 and after you got all gold eight inner gates runes + 4 quick runes).