crystals from a greased harpy???
I took both crystals with one ifrit sultan.

This is a bad method only because you need to get the ifrit, then upgrade it, and spend magic... but the harpy is upgraded along the main route...
Besides, it's not worth taking both crystals - the second one is useless; it's better to use that money to level up your hero!
As for taking them with angels and others... I saved the griffon lair for additional hero leveling... a small bonus + we don't lose troops, which means we get a little more experience, and we can also get experience at the altar...