Final answer:
Macbeth refers to the witches as 'imperfect speakers' because their prophecies are ambiguous and perplexing, lacking full explanations about his future titles, including that of king.
Step-by-step explanation:
When Macbeth refers to the three witches as "imperfect speakers," he suggests that their prophecies are ambiguous and incomplete. Macbeth recognizes that he is the Thane of Glamis, but the title Thane of Cawdor is already held by a living nobleman, making the witches' proclamations seem unbelievable. Their talk of him becoming king is beyond his comprehension since it's not within his realistic expectations or societal norms. He demands further explanation about the origin of their strange intelligence and reasons for delivering such unsettling prophecies on the desolate heath.