I forgot the 3rd category, those who see the world exclusively through the functional programmer lens :-D
Mapping functionally it worth it if the transformation between A and B is regular enough. But things are dirty. Some irregularities appear and now the perfect higher-order function doesn't work nicely anymore. With the imperative approach it just means adding 1 -3 more lines.
I find that an ideal set of tools allow us to describe our task and write our code using the same idioms.
The fact that "mapping JSON" doesn't actually involve calling "map" on JSON data is a great example of why not having generics in golang is frustrating for me.