If Demigryphs are undercosted then it is by no more than 10 points. 15-20 points is far too much. If Demigryphs were 70+ points per model it would be stupid to take them.
3 demi = 174points.
That's 9 wounds
3 strength 6 attacks (charge)
9 strength 5 attacks(with AP)
Basically 12 attacks at strength 6ish
9 Inner Circle knights = 225
9 Wounds
9 strength 6 attacks (charge)
5-6 horse attacks at strength 3
Attacks are roughly equal to demi's once you adjust for strength
So by these rough calculations the demi grpyhs should be 75 points each based on wounds and attacks. obviously there is some give and take with points ratio's since demi-gryphs are in special selection, can be one-shoted, etc... At toughness 3 I'd say they were 10pts undercosted.
But now they are toughness 4.... lack hatred for their good mount attacks. Which is why I now say 15pts.
Anyway, just thought I'd point out where I got my numbers from.
10-15 in a conga line to make your flanks unbreakable is probably their best use. lol.
Pretty much their only use, as far as I'm concerned.
On a more serious note.
I was actually thinking about this and I remember posting something last year about flagellants.
If you play scenarios with buildings, flagellants become very good. For holding or assaulting a building. Mainly because there is no protracted combat so you get the flail bonus every time! heheheheh. Most people don't realize this because most people don't play watchtower and/or treat buildings as impassible. If you get toughness buff on them then they won't martyr but you can keep a war alter close by to give hatret.
Blood and Glory scenario(the one with the banners) is also a good battle for them since victory points don't count. Flaggies at 11pts a piece become a very expensive throw away unit!!!
And them being a unit that usually dies is a big detractor to taking them in small units.
So tactically on paper: watchtower, blood and glory is when flagellants shine.