The format of the api (and its' payloads) also depends on the purpose of the api.
If the api is meant to be tightly coupled to an app, and is designed for that specific purpose, there's no reason to use anything like xml (or even, json) - it should be compact binary and minimal footprint.
If the api is meant to be general and apply across a range of client environments (e.g., you're selling this service api, and you dont know how or what your customer might use it for), it makes sense to make an api that's using an open format (like json, or xml) and have easily able clients in multitude of languages.
If the api is meant to be tightly coupled to an app, and is designed for that specific purpose, there's no reason to use anything like xml (or even, json) - it should be compact binary and minimal footprint.
If the api is meant to be general and apply across a range of client environments (e.g., you're selling this service api, and you dont know how or what your customer might use it for), it makes sense to make an api that's using an open format (like json, or xml) and have easily able clients in multitude of languages.