After losing 0 demons and deleting 7 demons to get a free slot I realized what makes me unhappy (yes, again ): good play is not rewarded, I could lose those 7 demons in fights and still have the same party. That's especially painful for demons who require items to be hired (Faerie, for example).
So how about giving some currency for deleting a demon? It can depend on number of levels the demon got while in the party, thus it would not help much if you hired a demon, copied its ability and disbanded right in next fight. Alternatively when deleting a demon because you already have 9 demons, the recently hired demon might get a free use of "Copy Ability" from the about to be deleted demon. The latter approach is probably better because this way you still cannot copy anything to you or your old demons so linking/disbanding farming does not make your party stronger.
Example: You had 8 demons, then hired Ogre so you have to delete someone. If you delete Gandayah, you can copy one Gandayah ability to Ogre for free.