A few more thoughts...
The Loremaster PrCl is a fairly good example of how to properly write a PrCl, although I think it is not perfect, in the sense that it could have more specials linked to spellcasting instead of other, but that's off-topic. The several spells and skills requirements (although the Wizard can reach them more easily) implies that your character is already focused on what the PrCl is supposed to improve much more than the progression in the core class. This is very good by itself, and I agree that only the Wiz/Sor that are really interested in that will be take this PrCl. Anyway, given the sheer amount of PrCls around, I have NEVER seen a single Wiz/Sor not aiming to a PrCl which fit her interests, and none thougth of going to be simply a Wiz/Sor for all her 20 levels.
I really wish there was something more than the Familiar in the 2 core classes to make the choice of staying coreclass a valid choice, and not a loss. I don't think that the +1 spellcasting given by PrCl is the problem, it is instead what makes the choice of a PrCl very viable since reducing spellcasting progression may hurt much more than the special PrCl features are worth. I hope I make myself understandable: I want BOTH the choices of taking a PrCl and of staying core class to be possible good choices; in the case of Sorcerer and slightly lesser Wizard, they are uneven. I am not very fond of bonus feats, but more of them could have been the easiest way to improve them while keeping them generalists.
Meantime, I think many of the FRCS PrCl are poorly designed. I don't own the book myself, so I apologies if what I remember may be very wrong... I don't like 5-levels PrCl, because they don't motivate a charater to consider herself a real member of that PrCl, and instead tend to represent a brief deviation from the PC's general progression, just as taking a couple of levels in a second (or third...) core class for few boosts in proficiencies, abilities, etc. Furthermore, with so few levels, they tend to be a bunch of scattered abilities with limited progression. All the various Arcane Devotee, Divine Disciple, Harper Mage... (I really don't remember them each, I just try to recall my impressions when reading FRCS!) seemed to me to grant possibly 1 or 2 unique abilities, the rest being bonus feats or anyway bonuses which don't distinguish them from other chars and could be achieved with feats, skills, or core classes abilities. Archmage and Hierophant were different: they really ARE a very horizontal-progressed deviation from the PC's advancement, so that you clearly take a single level of them and get one single special ability otherwise unavailable, and it makes sense that you can get a level in it sometimes (the 5 levels have no differences with each other IIRC, and they could be 10 or 20, "5" works just a general limit to how many special ability a PC can get in her life).