Pathfinder: Item-Free Ability Score Advancement

I have always been greatly frustrated by D&D 3.5's reliance on attribute-boosting magic items. The problem, in my view, is that such items are effectively mandatory. The game rules seem to assume that you will have and use them, and even if that weren't the case, they are simply so good that every character will want at least one, and usually two or three. I feel that anything so obviously useful should simply be built into the system, freeing up resources for character development in interesting directions. When Pathfinder proved to have the same basic problem, I created this system to address my pet peeve.

Alternate Ability Score Progression
The rules changes under this system are relatively simple, but have profound effects on character advancement in Pathfinder.

Magic Items That Boost Ability Scores Are Banned
Ordinary (non-artifact) magic items cannot provide any sort of constant effect bonus to an ability score. Artifacts are still able to do so, but DMs are cautioned to consider reducing any such bonuses to +2 or +4 for balance reasons.

Characters Advance Ability Scores Quickly
All characters may now increase two different ability scores by +1 at every even-numbered level. This is four times the normal Pathfinder progression of +1 to one ability score every fourth level.

Low-Level Ability Score Boosting Spells
Low-level spells which provide an enhancement bonus to ability scores are problematic in this system. These spells would normally be rendered relatively useless by the magic items which also provide a non-stacking enhancement bonus to ability scores. However, under this system, those items do not exist. Therefore, the effectiveness of these spells must be capped. Any spell or power that creates an enhancement bonus to one or more ability scores is subject to a cap on its effectiveness. No enhancement bonus granted by any spell or power can cause an attribute to exceed 16 + spell level + ½ caster or manifester level before other bonuses.
 * Example: A bull's strength spell cast by a 5th level cleric could raise a character's Strength score to a maximum of 20 (16 + 2 + 2.5 rounded down). A barbarian with Strength 18 would have his Strength raised to 20. If the barbarian were raging (+4 Strength), his Strength would increase from 20 to 24. If the spell's caster were instead 8th level, the cap would increase to 22 (16 + 2 + 4) and the barbarian would gain the full benefit of the spell and have Strength 26.