Joel,
I wouldn't hold up Ninco plastic pinions as a standard for excellence. I've had and seen many split ones even though they have a collar. And the awful Fly plastic pinions that Screwneck mentions also have a collar. I'm of the opinion that the type of plastic used is a big factor and possibly even the colour of the plastic. Back in the height of the Scalextric "split pinion" era some C7 Minis came with a grey pinion instead of white - I've yet to see one of those fail!