Defenders of the pronoun

Recently, for whatever reason, I've been in situations where the question of the status of pronouns has come up. Inevitably, there are people who vehemently reject the idea that pronouns are just a special kind of noun. One correspondent writes,

"Why insist on pronouns as a 'special case of nouns' when current handbooks from Hacker to Troyka and Ready Reference unswervingly place pronouns in a different category from nouns?"

Well, because they generally:

1. signify the same range of concepts
   
2. both are subject to distinctions of case
3. both are subject to distinctions of gender
4. both are subject to distinctions of number
5. share almost all the same functions (e.g., subject, object, determiner)
6. share the same set of modifiers
   

Actually, 6. is a bit of a stretch. Typically pronouns don't license determinatives or adjectives but they sometimes can in a pinch (e.g., the new you). Then again, proper nouns don't usually license determinatives or adjectives either and nobody wants to set them off on their own.

These defenders of the pronoun inevitably argue that if you look up "parts of speech" in any reference, you will be told that pronoun is one. It has always been so, they say pointing to the etymology (and falling foul of the etymological fallacy). But they offer nothing beyond tradition to explain why pronouns should get a class all to themselves. Nor can they explain why, if pronouns should, auxiliary verbs, for example, shouldn't.

These people are typically unsurprised that the physics, biology, and chemistry they studied in high school is no longer up to date. But they get positively defensive when somebody suggests that grammatical description has moved on. Why is that?