In retrospect, I'm not sure if my dreams were actually more vivid (or if the lucid ones occurred with actually greater frequency), or if they just felt that way because I moved so quickly between sleep and wake, so the dreams weren't cleared from my working memory. (If I had to wager, I'd say it's more a matter of recall bias than actual improvement in dream quality.)
Perhaps it takes a long, long time to adjust to a polyphasic sleep regimen. Perhaps some of us just can't do it, due to the idiosyncrasies of our physiologies.
An interesting article by a fellow HN commenter, who did an extensive self-study on sleep quality (including thoughts on the costs vs. benefits of polyphasic sleep):