I just finished a playthrough with a lot of single...
Naturally I wasn't attracted by single. I know it was bad but I still tested it completely. I have understand a lot of thing.
Ascertainement.
1) I am a single Priest.
2) I have 2 spells to cast (ressources) by spell level (1 to 9).
3) I have more spell to choose than multiclass each level up.
4) I have the same number of ressource (2) than multiclass.
4) But number of spell is opposite to number of possible uses. (If I have 999 differents spells and only 2 uses...)
To conclude, there is absolutely no argument to take a single priest. Each level of spell don't changing the world. (was not the case with devotion for the faithful at 20 accuracy. Got sooner = G.G. = eventual interrest of single class)
Few idea to improve single class :
1) Casting time / 2 for single. 6s of casting ? 3s of casting time ! You can buff far quicker ! Good !
2) +1 spell ressource by spell level. (2 now = 3)
3) Reworking the level 8-9. Awful.
4) Two uses of Empower on encounter.
With that : I buy it. A single priest worth it. If not : poop.