Such mechanics often exist for PVP spells, and other games have something like that for PVE as well.
Actually 'dimishing returns' work the other way around too, as some games prevent players from perma-crowd controlling mobs. If you CC something for too long or too many times, your CC eventually stops working completely.
I don't know how that could be implemented into Istaria though. Probably requires a lot of re-coding the game...