In a post about ability scores, Greywulf writes: "Earlier Editions of D&D recognized the synergy between Attributes and Classes by giving an XP bonus for having a high Prime Requisite stat. It didn't make a lot of sense to me – after all, a low Strength Fighter would have to work harder to defeat the Orcs, so to my mind should get more XP than the high STR Fighter for doing the same work."
The disconnect he's experiencing, of course, is because he's interpreting experience points as knowledge or practice. If you interpret it that way, you have to change a couple things, like dropping experience modifiers based on ability scores. (Unless, of course, you interpret the penalty as an indicator of how much harder you have to work to get the same amount of experience as someone better than you...)
I, of course, interpret experience points as reputation and confidence, as I've mentioned before. So for me, the XP bonus for your prime ability score represents the fact that people find it more believable that a brawny fighter performed mighty deeds than if a scrawny fighter made the same claim.
The sad thing, for me, is that the secondary and tertiary abilities fell by the wayside. A smart fighter, in OD&D, gets a minor boost to experience from Intelligence and Wisdom, making it possible for even a Strength 3 Fighter to earn that coveted +10% with Int and Wis 18 (depending on how you interpret the rules on this...)