The way the gym rewards work makes it pointless trying to help your gyms and putting Pokemon in to defend them after collecting the defender reward.
Example you manage to put 3 Pokemon in 3 different gyms and collect defender bonus. For the next 21 hours anything you do in gyms will be a waste as you can not get another defender reward for next 21 hours. The chances of anyone keeping defending a gym that long is next to impossible especially as there is nothing to make people want to keep gym level up to make it harder for opposing teams to take over.
It is easier to just fight gyms when you can get the defender bonus then wait 21 hours and do it again. It will mean using far less potions and revives that way instead of easing them.
The whole gym system currently is garbage.The turn over rate on gyms is rediculous because it takes no effort to take gyms away even if you are significantly lower trainer level than the current owner and that's even before the fact that several trainers can get together and gank a gym.
Training a gym is a stupidly long grind. A grind that is undone in a small fraction of the time and resources and much lower difficulty in general.
Controlling 10 gyms gives such a small bonus that it is barely worth doing in the first place and there is no bonus for holding the same gym for a period of time or making your gym a higher rank.
Even the XP rewards for fighting gyms or holding them are trivial to non-existent.
Gym pokemon need much better AI (higher % of their max potential dps). Training gyms needs to be less painful. Ganking gyms needs to go away entirely. And the reward for holding a gym needs to be more significant. IMO you should get paid for holding a gym for a certain period of time (24 hours for example) but only need to hold one two gyms to get the full bonus. You should be able to put as many of your pokemon into a gym as there is room for (not just 1) as well.
Yes ok if you could it would be easy 10, 20 or whatever coins a day every 21 hours so would be nice to. But not worth using loads of health potions to do when you get same reward if you beat same number of gyms every 21 hours and use less health potions that way.