Here's the story.
The majority of the albums were mono only originally. The only "official" version of all but the last three albums (IIRC) is the mono version, as the mono version is the only one the Beatles had any hand in mastering.
Many of the early stereo versions of the albums are awful -- undynamic, vocals panned hard right, drums hard left, etc. It's felt by many that the mono versions are both better-sounding and more authentic. Mono isn't terribly bad, anyhow.
It remains to be seen what the remastered early albums will sound like in stereo. I hear a good deal about them being based on the 1987 versions, which I find reasonable enough.
All I know for sure is that the version of "Revolver" I heard today was fantastic. It sounded like a modern record.