I once had to write an OAuth client for each of the major invoice/accounting cloud services. It was a huge hassle, with very little standardization. Everyone had to add their little quirk to the system (like client-specific endpoints, or some quirky encoding, or only accepting HTTP headers and not POST fields), depleting all the value of using the OAuth "standard," leaving only a bulky OAuth library. Simple GET/POST fields are much easier and can easily be transparently maintained in a few lines of code.