When you say free software, you have to clarify, 'free as in freedom, not as in beer', libre doesn't have that double meaning.
The problem is many people in the free software community enjoy confusing their audience and then take delight in pointing out their clever wordplay.
No reasonable person would decide that the best way to cut through a complicated overlap of a technical discussion a moral issue with many subtleties was with a pun, of all things.
Libre doesn't have a meaning at all, in English, so you'll have to explain that as well. In other languages that I know there is no such problem because the two meanings of "free" are clearly disambiguated by two different words.
The point is: with 'free', people might think they know what it means (it doesn't cost any money), 'libre' is context-specific.
I don't have my OED to hand to check, I'd be surprised if it wasn't in there though. I had come across the word prior to my introduction to free software, I'd come across gratis also.
Not that it disproves you, but the only reference I could find in the online version is "vers libre", meaning "free verse", of course. I don't have a paper version to check, and I'm not a native speaker, so I'll have to trust you on this one :)
In English 'libre' does mean free, but it means free as in 'free will'. It's not the same meaning as 'libre' in other languages. So it is confusing.