Clashing paradigms

For one thing, classic dispensationalism is a whole system of theology. It has a great deal of internal coherence. A system that is carefully and thoroughly elaborated, whether right or wrong, will almost certainly include answers to standard objections. And different parts of the system “come to the aid” of any part that is challenged. To a certain extent this is true of any system of theology. Vern Poythress, Understanding Dispensationalism, chap. 5.

Although this has specific reference to the Dispensational/Covenant theology debate, it has many parallels, viz. evolution/creationism; atheism/Christianity, Catholicism/Protestantism, Calvinism/Arminianism, Calvinism/Molinism, Calvinism/open theism. 

This is why coherence is an insufficient criterion to assess a paradigm or adjudicate between competing paradigms. It's often possible for a given paradigm to fend off objections by resorting to ad hoc postulates and stopgap harmonizations. Paradigms can be very flexible, and have the internal resources to deflect stock objections. 

In addition to internal consistency, there needs to be some evidence that the underlying claim is true, before a defender resorts a hypothetical explanation to fend off objections. Moreover, it's good to have evidence for the postulates and harmonizations. If not direct evidence, at least evidence for analogous situations. 

Special pleading ensues when a proponent defends a claim for which there's no good evidence, or even prima facie evidence to the contrary, by appealing to fact-free possibilities.