I just returned from our cruise on the Spirit so I am in a position to answer my original question as to Ballroom Dancing.
We found that there are great dance floors on this ship, much bigger than the Wind or the Dream and other ships that we have sailed. There was a lack of available music however. On previous cruises the Norwegian Showband would play for 45 minutes prior to the show each night and another set before the second show. On the Spirit, they did this on the first night only.
We did find some nights when there was excellent dance music in the Galaxy of the Stars lounge on Deck 13. There were three main groups that provided music, Fusion Wave from the Phillipines, Millenuim from Barbados and the Jazz band that consisted of many members from the Showband. The did provide excellent music however there were nights that we could not get to the location due to conflicts with dinner or the show. On one evening, the Jazz band had a guest soloist who did a
Sinatra tribute. Two days later, they did a set with a female vocalist named Sara Cornell who was fantastic. She is a gorgeous, tall (6'2") blonde who did many jazz and blues standards with the band. It was truly a memorable concert.
On the last night aboard, the Jazz band did an early evening set with all ballroom music including Rhumba, Cha Cha, Tango, etc.
If you are looking to dance on this ship make sure that you make requests for individual songs or dance styles since they can adapt their sets to accomodate the crowd.