To me, deductive logic is essential not just for distinguishing between foundational and coherent knowledge, but to any sort of reasoning. For instance if you want to really figure out (reason) whether a certain proposition (or a piece of knowledge) coheres with your previous beliefs, you have to do some sort of deductive, abductive, inductive reasoning or a combination of two or more. For instance, let's say we want to evaluate the truthfulness (truth value or whatever else) of a proposition :
"High scorers on the SAT are intelligent"
- The SAT measures intelligence
- (Derived from #1) The SAT is devised such that it measures what it intends to measure
- (Derived from #1) A high score on the SAT indicate a high level of intelligence
#2 and #3, while not explicitly in the form of
- "Socrates is a man"
- "All men are mortal"
- "Socrates is mortal"
they still rely on #1 to have any truth value, and are hence part of deductive logic. So one must practice deductive logic in order to accept or reject a particular statement.
(Addition) I looked up "deductive reasoning" just to be sure, and the above (Derived from #1)s are examples of modus ponens; i.e. classic deductive reasoning. Then what is the purpose of distinguishing coherentism from foundationalism if they are so intertwined in actual use?