Jimmy was "a bit unknown" primarily in the United States. He was very well known in Europe and hugely popular in Japan. He wasn't really the "perfect fit" for Wings. It seemed a good opportunity at first, a chance to go some good touring and play large venues with a name musician who would draw crowds but the tight control Paul had over all aspects was stifling.
I think people forget how incredibly uncool Wings were in the seventies. McCulloughs joining of the band showed an astonishing lack of ambition and imagination by a young guitarist thought to have had a bright future. In an age when guitarists were still highly regarded as artists as important as any front man, to become a jobbing player on a wage for Paul Bloody McCartney was career suicide. As far as his credibility was concerned he might as well have joined ABBA's backing band, or taken a job at the bank.
If McCartney didn't treat him right then I have no sympathy. He shouldn't have been there in the first place.