Nonetheless, most contemporary American dictionaries present without comment the meaning—and the pronunciation—you abhor. The history of English is a many-centuries-long tale of the borrowing and adaptation or corruption (take your pick) of words from other languages. By now, of course, many more Americans are fluent in Spanish than in Latin. And as far as I can tell from my Spanish, French, Italian, and Portuguese dictionaries, none of the major Romance languages—languages directly descended from Latin—have retained either bona fide or bona fides, in its original sense or any other. The misunderstanding and consequent change of meaning in English is only to be expected.
It seems to me, then, that we should draw the line here: Let's accept that bona fides has become a noun meaning "credentials," and bona fide an adjective meaning "authentic." But let's fight tooth and nail against a tendency to mutilate the adjective. Uses like the following, from The Oakland Tribune, the Calgary (Alberta) Herald, and the St. Louis Post-Dispatch, respectively, are a little too common to be just typos: "He's a dog with a mission. A bona fied working dog …" "Junior oil & gas companies stepped back to the main stage last year, becoming bonified headliners in their own right." "Jaskowiak … is a four-year starter and a bonafied All-Metro prospect."
R. H. FANDERS, of Council Bluffs, Iowa, writes, "Last Sunday on CBS this sentence was used: 'I was better than her.' An hour ago, again on TV: 'I was wondering if this time my dog did better than me.' Than is a conjunction, never a preposition. It ties two clauses together. The verb is understood in the second clause: 'I was better than she (was).' Am I being old-fashioned? Have the rules changed?"
It depends on the level of formality you expect from TV shows. You're right about the traditional view of the grammar of such sentences. Conjunctions tie together things of the same kind, including clauses; prepositions attach nouns, or elements functioning like nouns, to clauses; and than is supposed to be a conjunction.
But is it "never" a preposition? Consider "She's the one than whom I was better." That is to say, "I was better than she"—so what's "than whom" doing there? "Than whom I was better" is grammatically equivalent to "I was better than whom," which is grammatically equivalent to "I was better than her." If you insist that than is a conjunction, "than whom" would have to be "than who." But I don't think any of today's authorities on language would make that "correction," and very few from the past 200 years would either. Sometimes, even in formal English (than whom sure ain't colloquial), than functions as a preposition.
The main reason not to welcome all prepositional uses of than, in my opinion, has to do with sentences like this one: "I like her better than him." That's clear, no? It means I prefer her to him. If we start allowing than to be either a preposition or a conjunction catch-as-catch-can, soon that example will become ambiguous: do I prefer her to him, or do I like her better than he does?