I really would like to know myself how to better answer such kind of questions.
And this skill is very important in requirements formulations and communication with clients/customers.
Infopath is client technology to be run inside and in context of or by some other "container" like Infopath Sharepoint Form Services, Infopath Filler, Office apps like Excel, etc.
IMO, there is sense to make APIs to the latter (server-like processes, tools, frameworks).
To which of them are you planning to run Infopath forms and respectively to get APIs to?