I think that one note solos sound fantastic in context. The harmonica playing a rhythmic single note just sounds right. Plus it's an accessible way for us learners to contribute to a song. I just heard this - Rod Piazza with the Dirty Blues Band playing Hound Dog. The solo starts after 50s. Yes, ok it's not quite one-note, but based around rhythmic root notes. Later on he lets fly with many notes.
