I spend money in coins because the old Gym system was unfair, people puting like 30 mons to have they rent of 100 + 5000 stardust with out effort per day and kick out the chance for other players to have at least 10 coins and 500 stardust just because of greedy.
This new one is fair, but pointless because there is no motivation to take down gyms, cuz players don´t help rival´s team to collect. Thats what i see.
I spend in coins for incubatiors, but only if i managed to get nine 10K eggs, so i do once in a while. But this new staff of raids is very interesting, so i need more coins so... pay for more coins.