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I knew that defending a gym gave me some badge exp but not how much so seeing this makes me feel less bad about some of my stable gyms never letting my Pokemon out.
This is good news because 4 weeks of idle defending means I get a free gold badge. I'd say a free gold badge of a gym I don't have to walk every day to is worth not getting the coins. And just visiting the more active gyms every day and doing a raid battle every once in a while means that I am making meaningful progress on those gyms too.