The factory slots are otherwise essentially worthless once you've squeezed out the few useful four stars it produces. The chance for a five star is so low it's not even worth the oil*. The core consumption's not bad for focus leveling a tank. It's not like the "pay to not play" insanity that is level skipping portal of chaos for more than the potential reward or spinning the wheel of disappoint for way too many cores.
* Like, I'm pretty sure you have a greater chance per oil of getting a five star tank running maps in zone five and six than building tanks in the factory.