Just read all the posts. Interesting...I have to say I never even heard Felder sing solo until he was on Hager's tv show. I never followed him. Although he's a great guitar player, he'll be the first to tell you, he just turned me off with his "I had to..." attitude. To me his solos were better after Joe, as he said, taught him how to phrase them. He and Joe played together for a while when the Eagles broke up the first time.
I think he may have been somewhat intimidated by Joe, not so much his playing Bernie' parts on the guitar but all the other things Joe did that he didn't. Joe played a lot of acoustic guitar especially 12 string. That helped Glenn when he was singing. It also gave the band a broader acoustic sound by having a 6 string and 12 string vs just one or the other. Other than the opening to HC I've only seen Don play acoustic in one other song.
About the time Joe came in, Don and Glenn we're thinking that they'd like to have an organ on some songs, Joe plays organ. When he first started you also see him playing the keyboard. This let Glenn step up front when he was singing. Don did play mandolin and steel guitar. Stuart plays Don's part now and plays the mandolin part on guitar, although I believe Stuart plays mandolin.
Did Don contribute to the Eagle's, I'd say yes. But I also think that when he started complaining about everything i think Glenn and Don may have looked at his contributions vs his complaining and his lack of diversity and decided he had to go.