I'm becoming a bit obsessed with the lower than usual leadership for the Beasts of Chaos Army and I was just looking through the magic items to see if there's anything useful to help mitigate it.
Roll on the Horn of the Great Hunt, it seems to me that this is an absolutely must have for an army using the ambush rule - I'd call it an American Express item ! If you know what I mean

+1 leadership and Foe Renders in each unit improve the likelihood of a unit appearing for the ambush by about 73% !
It goes from 41.66% to 72.22%, that's a heck of a leap.
Why might you not use the Horn of the Great Hunt. The only reason I can think of is if you;re not ambushing very much (or at all) or you absolutely must have a different combination of magic items to fulfil your tactical plans. It seems like a steal at the price.