Franking Credits., page-139

"Marginal Tax Rate" is simply a term used to describe the rate of tax a certain tax payer pays for every additional dollar of income earned.

Your beef seems to be with superannuation being concessionally taxed, not refundable franking credits. I'd suggest you are commenting in the wrong thread?

Super is taxed concessionally because negatives such as contributions being mandatory, funds not being able to be accessed until a certain age etc meant some carrots also needed to be dangled. Keating understood this. Morrison wound this back somewhat by introducing the transfer balance cap, limiting the earnings taxed at 0% in pension phase.

Cheers!