 Richard Garfield, in his book Characteristics of Games, actually made a very good point about definitions and how nerds, especially in gaming, like to be really fiddly about them. But he said, when in doubt, don't be prescriptive unless you're doing academia. Be descriptive. So instead of trying to write a definition of what is formally a worker placement game, describe what the hell a worker placement game is and then work with that description. He'll be a happier person if you do this generally in your life.