Ken Udas once challenged me to argue for "Free" (after writing Say Libre).

At the time I did not have time, so, I sort of copped out by referring everyone to RMS's writings.

However, it is a good exercise, and I invite people to populate and extend this table with arguments for/against and counter-arguments:

Free	Libre
The word is familiar in English	There are many loanwords in English and "libre" is a useful addition.
In such contexts, someone will have to point out the real meaning, and then people will get it, and not make the same mistake again.	The word "free" is ambiguous and in some contexts this causes real confusion. Even after explaining the difference, confusion can persist, and it has to be re-explained every time there are new listeners present. Perhaps we should not use the word "free" at all in this context (use only "gratis" and "libre")
When searching for "free" resources, irrespective of the intended meaning, people will also find and learn about free/libre resources.	This strategy will ensure the confusion persists, and is bordering on devious.
"Free" is consistent with its use in the context of free software upon which the free/libre knowledge concept is based.	Libre is now an accepted loanword in English and is more precise and unambiguous.

Libre vs Free

Resources