JBD,
I agree with your statement except for the part about other OCPD's between the series rated combination.
The series rated combination is tested with those two devices. If other fuses or CB's are placed between the series rated combination devices, then that adds additional dynamic impedance that was not considered during the UL testing.
The main reason to require a tested combination, is dynamic impedance.
See the following article:
http://ecmweb.com/mag/electric_dont_felled_higher/index.html
