What does adding dark rum give to a fajita stir-fry?

In the Caribbean and other Rum producing areas, you find Rum in all sorts of recipes

It is sometimes an unusual flavour pairing, but not necessarily bad. In savoury cooking it adds a semi-burnt sweetness, more like toffee or butterscotch than straight Rum

Caribbean and Mexican cooking is well blended and harmonious with many common ingredients (chilli, coriander, avocado etc.)

Considering the popularity of spicy jerk meat and Rum, the Rum Fajitas would be an interesting experiment

It adds the flavour of rum.

I'd love to go into a detailed exposition of how rum is a fantastic pairing for "latin spices", but as far as I can tell, no such relationship is documented. It's just a theme ingredient, something that the author evidently felt would make an interesting combination.

Rum is more commonly associated with sweet fruits, chocolate, and apparently certain dairy products. It's not something I'd consider adding to the typical fajita mix (peppers, onions, cumin, chili) - seems like a waste of perfectly good rum to me.

If you don't want to spend the money on rum, just buy some rum extract in the grocery store. It's not as good as genuine rum, of course, but as a first-time recipe experiment it will give you a pretty good idea of whether or not it's worth using real rum the next time (assuming you like the recipe at all).

Any kind of alcohol added to sharp spices like ancho, cayenne, chipoltle, or even paprika, will bring out more of their spiciness. Alcohol, in it's most raw state (Think cheap vodka) is bitter. The five tastes that we can perceive are sweet, sour, salty, bitter, and umami, these tastes play off each other and can react with certain ingredients. Any bitter agent in a dish will make it taste more spicy than it is. Where as something with umami flavor (Think red meat) will lessen spice. Of course these rules can be broken, but what rules can't? Especially in cooking. As far as rum goes, it is a strange pair, I might just add a dash of the beer or wine I already had open, but who knows? Only one way to find out.

Alcohol is a better solvent than water for many chemicals, in particular for "non-polar" chemicals. This still works when mixed with water (as in rum), as alcohol works like soap in that respect. But unlike soap, alcohol doesn't add a significant flavor of its own.

Rum is of course not just alcohol, so you do get those flavors as well.