This is known as the "odd man out" syndrome. I currently work in a medium sized company who are doing a huge ERP switch over. I'm a front-end developer by trade, but know .Net as well. One part of the contract stated our company needed to have X amount of company resources (people) to have on the project.
The downside is I hate my job now and am actively looking to get out of here. They told me recently after the release, I'll be one of the ongoing "resources" to help manage post-release defects.
So I agree on your last point as well. It's not fair and unfortunately, it's a no win situation for the developers. If I do a shitty job as a JDE developer, they get pissed and might evenutally fire me. If I do a good job, then I get tasked with all kinds of stuff I have neither the want or desire to do.
Six months in and I hate working in JDE but all the contractors think after two weeks I should be a pro with it since I'm a "developer".