That is a very good point.
In fact, it wasn't until this week that I learned quite how much of the Abhidhamma (and even perhaps the reason for its genesis) was in response to the tradition of Indian Philosophy which asked questions of early Buddhists, many of which the Buddha did not think required an answer - presumably as they did not meet with his teaching standards as outlined in the Simsapa Sutta.
Yet, the questions were asked nonetheless, and the Indian philosophers wouldn't accept noble silence as an 'argument'. The Buddhists would lose face, be accused of not being able to reply (and thereby having their Dhamma "defeated"), and were therefore compelled to frame positive arguments, which they endeavoured to keep consistent with Buddhavacana. It's an interesting thought to consider isn't it - that perhaps the commentarial and Abhidhamma traditions arose not for the purpose of serving the internal interests of Buddhists, but to defend it against irrelevant questioning from outsiders (almost like an extension of the Digha Nikaya). Even in the Buddha's time though, these sectarians were present, and arguments often had to be framed in terms they could understand, in the interests of communication and understanding.
The risk we run is that those whose teachings the Buddha parodied, would later re-infiltrate the teaching as various explanations were imported from the broader Indian Philosophical/Religious tradition (take 'heart-base' as 'mind-base' for an innocuous example, or 'knower of lokas' which ven. Buddhaghosa depicts over several pages as an Indian cosmological understanding... though there are more serious imports, pertaining to matters like consciousness and perception) and the nuanced meanings and explanations of the Buddha get lost in the process under the scaffolding of beliefs, views and explanations.
(And just because I predict someone will accuse me of rampant speculation here, read Ediriwira Sarachchandra's "Buddhist Psychology of Perception" for more details... it traces the evolution of the Buddhist psychology of perception from the earliest suttas through to the latest sub-commentaries, explaining much of what was added, imported or discarded along the way)