You forgot a certain clause in Ability Advancement.  "Special: You can gain this feat multiple times, its effects stack."  Six feats for +10 to whatever ability score.  In comparison, the Dragon Disciple is horribly constrained in what it can do.  The only difference is one prerequisite feat for an uncapped pool of ability advancement.  And Dragonfire Adept natural armor<em>also</em>stacks with enhancement bonuses to natural armor, just like Dragon Disciple natural armor.  Compared to the options available, Dragon Disciple just isn't very good.  You can get equal or better options elsewhere, and the way these rules work, trading a level to save yourself a feat or two really isn't worth it.<br/><br/>Some of the partial-casting/manifesting PrCs, though?  They can be nice with these rules.  I should know, I'm already taking Sovereign Speaker abilities.