When RMS is giving a speech, he has an opportunity to expound on what's now wordplay about "free software", to a captive audience that's perhaps already receptive or prepared to listen.
But low-level grassroots advocacy opportunities often happen in contexts in which people are talking for some other purpose, and if you only say "free software", and they don't already know what you mean, you're actually working against your goal. People who don't know what "free software" means naturally assume you mean software for which they don't pay money. If you instead say "libre software", it's not misleading, and if they don't know, and they care, they can ask you about it, or look it up.
(I suspect it would've been better to fully embrace the "libre" term before an office suite was branded that. Now we have a new potential source of confusion, such as "Yes, I already tried Libre, but liked Office better". But I still usually feel more effective saying "libre software" than "free software". And, in practice, I end up saying "open source" perhaps the majority of the time, even when I'm thinking libre specifically, because "open source" is more established than "libre", perhaps because the FSF keeps saying "free".)