Complex sentence: To do ARP spoofing, the NIC must be kept in promiscuous mode, so detecting machines in promiscuous mode with no valid reason is an important defense against ARP spoofing.
Split into simpler sentences: To do ARP spoofing, the NIC must be kept in promiscuous mode. Hence detecting machines in promiscuous mode with no valid reason is an important issue in order to deal with ARP spoofing.