Yeah production plants are a decent investment to make. In real life countries might make investments in foreign currency, gold, or
infrastructure developments with some excess cash.
But since in SC theres a global currency, gold is too cheap, and infrastructure doesn't pay off. Production plants are a good bet to invest excess cash in. They are quite expensive so you wont need billions of them (unless your really rich), and are not in high demand for your country or at all if you have an enterprise (things like nukes, hospitals, and expensive weapons are expensive as well. But either can't be sold easily, or get used by your country when bought (hospitals, weapons), or require upkeep (hospitals (staff), weapons (staff, ammo)). Plus stockpiling production plants has a minimal effect on the global economy, sure someone might have to wait a bit longer for a corporation to be built or pay a bit extra, but it doesn't effect any supply lines significantly. Plus it makes people like me richer ;), (invested largely in production plant corporations)
Also in the case of a recession (surplus of production plants (meaning growth has declined)) you can pick up some cheap production plants, helping your self by making a investment with large return, and helping people like me who are biting their nails because they invested so heavily in production plant corporations and now no one is buying them.