Banquo is extremely suspicious about the witches' motivations for revealing these prophecies. He fears that their revelation may play some role in a deceptive scheme designed by evil forces:
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence— (I.iii.134-136)
The witches' speech could be completely benign, but Banquo and Macbeth have no way of knowing this. Banquo warns Macbeth to be on his guard and not to give too much credit to the witches' message. Dire consequences and untold repercussions could result from paying heed to the witches' unnatural words. Even so, the witches' prophecies linger in Macbeth's mind as he considers the potential of their words, despite Banquo's warning.
Banquo is naturally intrigued by the prophecy that his children will be kings; however, each time Macbeth mentions it he counters that the witches also said Macbeth would be king. Banquo is concerned because one does not follow the other: it should be Macbeth's children that become kings.
Banquo is also concerned that the witches are evil; he calls them 'instruments of darkness' and says that they cannot be trusted. This suggests that he is less susceptible to the witches' temptation than Macbeth. It is depressing therefore, that later on he does seem susceptible, 'May they not be my oracles as well / And set me up in hope?' He is now open to the idea that he also might benefit from the witches and their prophecies. He, like Macbeth, has been corrupted by the Witches.