Currently the Prayers are killer at forcing your opponent's hand when it comes to dispel and power dice. The more prayers you have flicking around the board the more they waste their dice. Killer magic defence.
However, the rules you're proposing would remove that role. Honestly, if that's the change I'd only use prayers on things like the Stank or Walter, maybe in challenges. And in that case, it's a glorified Lore of Beasts.
Not flaming, just my opinion: Single model spells really, really suck. I know the priests are currently overpowered, but it's too much of a gimp. Maybe make it double casting value affects the whole unit, but if it's a single model I'd leave the priest at home.