If the Chavadi was so awful, was there any upside for Amir to stay there for his rheumatism?

Yes, despite the dreadful physical conditions, staying in the Chavadi provided an immense spiritual benefit. Chapter 22 explains that accepting Baba's word was the 'Great Prasad.' The Chavadi's location was such that Amir received Baba's vision automatically and without effort. He could see Baba every morning during His alms round and every evening when Baba came to the front of the Chavadi. On alternate nights when Baba slept there, they were separated only by a plank door and could converse privately after other devotees left, enjoying a firm companionship with Sai.