Pros.
They can now join units of knights, or gain a lookout sir by being within 3" of a unit of knights.
Cons.
The knights they hang out with are now more likely to take casualties, so need spell support to keep them tough enough to keep fighting.
So, why not get the Lore of Beasts or Lore of Metal on him, and power up some Black Knights to accompany him.
Of the new magic items, I really like combining +2 or +3 attacks with Red Fury, especially as this can be increased a further 3 attacks with Lore of Beasts.
Then get rerolls to hit from a Vanhels or a Corpse Cart or a Hatred banner. Wow. Whole enemy units just... gone.