Not sure it would be accurate either to strictly call them metaphors for mental states, or to call them literal places, because they seem to involve a "location" for rebirth, but not a temporal-spatial location within this universe. On the former view, they seem rather convoluted as metaphors. The five realms, six realms, and ten realms models could all be seen as metaphors for mental states, but what exactly do the 40 planes of existence symbolically represent? Any metaphorical interpretation would have to be very convoluted. The second view, though, is also rather absurd, because it begs the question, "If they're physical, where are they?" which is what Stefan is asking here.
It is pointless to debate whether these realms are real or simply fanciful metaphors that describe the various mind-states we might experience in this lifetime. The real message of this cosmology is this: unless we take steps to break free of the iron grip of kamma, we are doomed to wander aimlessly from one state to another, with true peace and satisfaction forever out of reach.
The picture of the world presented in Buddhist cosmological descriptions cannot be taken as a literal description of the shape of the universe. It is inconsistent, and cannot be made consistent, with astronomical data that were already known in ancient India. However, it is not intended to be a description of how ordinary humans perceive their world; rather, it is the universe as seen through the divyacakṣus (Pāli: dibbacakkhu), the "divine eye" by which a Buddha or an arhat who has cultivated this faculty can perceive all of the other worlds and the beings arising (being born) and passing away (dying) within them, and can tell from what state they have been reborn and into what state they will be reborn.