(( SoonerSox,
The Griffon Banner could be useful against Vampire Counts, Undead Legions, Tomb Kings, and Daemons (anything with Unstable or Undead in their penalties)... could being the operative term.
As Fidelis said, with its point cost being 60, the only model who can take it is the BSB, and that puts him at a huge risk, for only a nominal (almost 0) gain.
If the Griffon Banner were 45 points, at least then Inner Circle Knights, Demigryphs, and Greatswords could think about taking it... but the banner's overall effect, plus its penalty, just doesn't make it worth it. At 60 points, it should just double your rank bonus. ))