The OP was "baffled" why anyone would ever use this API. Clearly, the API's success shows that sometimes this tax you highlight is well worth it. (That was my point as well.) Nobody is claiming the tax doesn't exist, just that it shouldn't be baffling that a rational actor would choose to pay the tax in exchange for the corresponding benefits.
(Not to mention with very minimal effort you can usually avoid the majority of the specific tax of latency you mention, by doing things like parallelizing the request with other work or doing it asynchronously to the user's interface.)
