In Watchmen, why did they make it Jon's watch that he had to go back for in the Intrinsic Field Generator?

Very true, the minimized role in the film did take away from most of the symbolism, but the fact that forgetting his watch is the reason that he no longer perceives time as we do is pretty heavy in it's own right. Also, the name of the book wasn't really referring to time (although that was a theme in the story) it refers to the characters, and how before the story that's what they were called. (this was implied, never stated) Another theme of the series was "Who Watches The Watchmen?" which WAS stated in the story, about how they'd gone from heroes to oppressors. The title had several meanings.