Melanthios is a character whose significance lies in the way that he is contrasted to other characters in Ithaca who have remained loyal to Odysseus and who support him when he returns. Melanthios has sold out to the suitors, and accepted that his former master will never return very quickly. As a result he is a negative foil to the other characters who have retained both their loyalty and their belief in Odysseus. Note, for example, what Melanthios says in Book 17 after he has broken every rule about entertaining strangers and those poorer than yourself by striking the disguised Odysseus:
Lo now, how the cur talks, his mind full of mischief. Him will I some day take on a black, benched ship far from Ithaca, that he may bring me in much wealth. Would that Apollo, of the silver bow, might smite Telemachus to-day in the halls, or that he might be slain by the wooers, as surely as for Odysseus in a far land the day of return has been lost.
He expresses considerable disloyalty and an evil spirit, as he rejoices in the death of his former master and then goes on to wish for the death of Telemachus, whilst also insulting Odysseus as a "cur." Admittedly he does not know the identity of his master, but even so, he shows himself to be a figure whose importance lies in the speed with which he has betrayed his former master and joined forces with the suitors.