Does anybody know if ordinary, working musicians (such as monks using those small keyboards I see in fine art) used the modes in everyday life? Or were the modes just theory known to a few?"

Yes. Just a few blocks from where I live is St. Mark's Cathedral (Episcopal) that has one terrific music program set up by recently retired Music Director Peter Hallock, an English countertenor who's been living in Seattle for several decades. Among other things, every Sunday evening, they hold a compline service that is attended by hundreds of people and is broadcast.   CLICKY. Chants, many—probably most—of which are modal. Also, St. Mark's boasts a monstrous Flentrop organ complete with 3,944 pipes, ranging in size from 32 feet to less than one inch. Lots of music during services, much of which is modal, and the cathedral is also used for organ concerts. Big enough to hanger a dirigible in, the space is most impressive, complete with Old World reverb!

On a somewhat smaller scale, in the mid-1950s, I discovered what modes were all about from an article in The Guitar Review, a high quality magazine that used to be published by the New York Classic Guitar Society and containing excellent articles on music theory, among other guitar-related subjects. The article said something about, beyond polyphony, the modes were never harmonized—chordal forms. I figured, "Why not?" and proceeded to work out triads to the modal scales in the same way that chords are derived from major (Ionian) and minor (Aeolian) scales. I started using them to accompany songs that I discovered were modal and it worked very nicely, fitting the songs beautifully.

Later, when I met Rolf Cahn, I learned that he had done the same thing. I don't know if Joan Baez worked it out that way, but on her early recordings at least, she uses appropriate chords for modal songs. Not everybody does.

I've found that, other than major and minor, I encounter Dorian and Mixolydian more often in folk music that any of the other modes. The Phrygian mode occurs fairly often in flamenco music.

You can listen to each of the modes HERE.

Don Firth