The stone for the stone soup, so to speak. Guilty as charged, but with an important purpose. We need the experience and to establish the working relationships. For example, Many who want to contribute have never done UNIX programming. It's not the same as C application programming on other platforms.
Cooperative development is an adjustment also.
Many probably don't even have a Linux box yet. Plus, we all can agree that it's one of the goals. However, PLC functionality is not the limit of what can be done. The shared memory 'core' and the routines to get data in to and out of them, probably do need to be written by a linux hacker. That 'core', as you correctly point out is the 'stone'.