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