Pax: Well, that is the theory at least, but in practice it doesn't much work that way.
If the entry requirements are themselves pretty much something you might have taken anyway, or at the least something you can get a good benifit from taking without sacrificing something of great importance, then having requirements aren't much of a balancing factor at all.
'Ergo', indeed. Exactly what is the basis of the conclusion that if only some people can take it perforce it must give you more advantages than training that anyone could experience? That is a design decision, not something that is forced on you by the self-evident logic of it.
And as far as specialization goes, specializing is itself always a means of improving ones power in an RPG (and to a certain extent in any resource allocation game and life in general). The specialist benifits from having some defence or attack of such extraordinary power that challenges he expects to encounter cannot resist or overcome it. He mitigates his weaknesses by using tactics to force his opponents to fight the battles that he wants to fight. The generalist has a much harder time, for while he doesn't have the weaknesses of the specialist - he almost certainly will be disadvantaged in something versus everything he faces. Even the party structure breaksdown along lines of specialization. Which party is more capable, the one with a 8th level fighter, 8th level rogue, 8th level cleric, 8th level wizard, or the party with 4 2nd level fighters/2nd level rogues/2nd level clerics/2nd level wizards? So it is not like you are giving up anything by specializing, the more so because in the case of the sort of PrC's that are actually taken they are specializations that PC's would have taken anyway whether PrC's existed or not.
Basically, if you have a problem with unbalanced PrC's throw out pretty much all of them with Fighter BAB progression or full caster spell progression. The 'broken' list is a sublist of that list and probably starts with OotBI, Templar, Red Wizard, Archmage, Frenzied Beserker, Weapon Master...