Hi,
I agree with you that SAML and OAuth can be a good solution.
They are more secure, standard, well documented...
But IMHO I think that they are acceptable for enterprise authentication (employee) or if you want to interact with external authentication services that are outside your network borders.
I'm talking about a easier use case. A guest captive portal that needs authenticate user on an internal backend, without using Clearpass provisioning workflow.
It this case, I think, that overload that comes from SAML (use bouncing between different page) or OAuth is absolutelly unwanted and unecessary.
I think that it could be problems with Apple CNA or Android CNA.
In any case, I agree with you... Best choice for me is open a RFE, but on latest documentation HTTP is already mentioned as authentication source (not authorization) so I should be better open a Bug fix request ;-)
I'm kidding. ;-)