I agree. Especially in the case of the "vancian" casters. If I'm not getting anything anything out of the tier 8/9 spells and I'm not looking to beeline to specific spell(s) ASAP then single class holds absolutely no value. And this is absolutely the case with Priest for sure 100% of the time. If Empower or Salvo get any nerfs in the coming patches it will probably be the case for Wizards too. Multiclass builds should be far more about specialization than they currently are. As it stands I think even the worst built multiclass is only neglibly behind the best built single class, and that single class builds can end up far worse than even that.
I think a good start to dealing with this might be giving Single Classes an extra ability point at level 1, buffing Prestige, and reworking Great Soul into more empower/encounter rather than the pointless thing it currently is. But that's just my gut reaction.