For the new classes, it seems like a bit of compensation as well. A new class has to face competition with classes that have a dozen or more different splatbooks worth of feats, spells, etc. and multiple threads worth of CharOps optimization tactics written up for them. If the new class (Dread Necro or Archivist, for instance) isn't strictly better than a core-only class, it's going to be dogmeat against a core class using splat content.

Then again, there are plenty of splatbook classes (Samurai, Healer, Warmage, Hexblade, etc.) that suck hard enough that they serve as counter examples.

I suspect it's not that splatbook material is overpowered, as that splatbook material contains a plethora of material, some of it overpowered, and some of it just as egregiously underpowered, but that the OP examples stick out a lot more than the feats, spells, etc. we look over and think, 'Worthless' before moving on to the Shivering Touches and Arcane Spellsurges.