Interesting question. I recently got an 11 course Baroque lute, and have done some reading up on strings then and now. There's still a lot we don't know apparently about what they used for bass strings and how they were made. One thing which makes sense is that gut strings as they get thicker (and duller sounding) for use in the lower registers, need the octaves to clarify the pitch. Wound strings, which were available sometime in the 1660's (though they took awhile to catch on) have a strong fundamental and don't need the octaves (to my ear anyway) to have the pitch sound distinct. However, there is tradition. I'm thinking of leaving the octaves off on the 7th-11th courses because I find it easier to stop the basses when appropriate without the octave string there. But most likely I will stay with the octaves if it still sounds better with them. Theorbos of the period often had single strings. I'm sure there is someone out there with more detailed information. I haven't heard evidence of Baroque lutes with single strings beyond the 1st and 2nd course, which is traditional.