I am renting out my cards for players and earning about 20 dec a day while I had rented some good cards to push my CP over the threshold to earn a few more chests.
I love the passive income I am getting right now, even if its not much at the moment, but the new rental system is really easy to use and understand. I'd much rather own cards too but my budget wont allow me to spend 500 dollars on a card while I can just rent it for a couple days for under 2. So while it makes sense short term/temporarily owning is definatley the way to go (because you can rent them out and earn $$).
Renting is really best for when you need some power to reach a league theshold or if you have a tornament; or perhaps you are returning after a long break and have a full capture rate to use up.
I've been playing a lot latley and my ECR is down to around 60% so this is a good time to sit back, put everything up for rent and let it charge up again while you earn a few dec's.