Why should wāw followed by an imperfect not immediately be called an exception?

Supplementary Practice 3: Rhetorical Judgment, Contextual Evidence and Combined Devices

• The wāw may coordinate clauses, or an underlying nominal structure may be involved
• Because every wāw before an imperfect must be circumstantial
• Because an imperfect verb always makes coordination impossible
• Because an underlying nominal reading is never admissible