See, I disagree with that entirely. Okay, well maybe not entirely - but I can find many places where it wouldn't work.
D&D isn't just about getting all the spells. It isn't about min/maxing. Maybe I am happy to have a character who delays his spell progression for 3-5 levels and gets some cool traits on the side. Sure, that defiantely means that my character is now behind in the spell progression. But maybe the character has an IC reason for wanting to take the stuff that boosts the turning. Or to take the ones that allocate more metamagic options.
I guess what I am saying is that spell progression may be what drives a character concept, but it certainly doesn't have to be. There is nothing wrong with being a level 18 character but not being able to cast the same amount of stuff that a straight cleric 18 could.
To me, PrCs are more about customizing the character to make them unique and fit what I see the character doing - not about just looking for the PrC that gives me the quickest way to a full spell progression and a bunch of cool stuff on the side.