Its great for getting beginners out there and getting them some easy, early success. There are some basic skills that can be taught easily on the knee board that transfer to skiing. When my daughter was 10 she was calling for the knee boards quite frequently. I knee'd (Ha!...get it???) to get that thing out more this summer.
Anyway, she was motivated to get out on the water and get behind the boat, she was gaining confidence, and she was having a lot of fun. These are all positive things.
One of my work colleagues who knows I am a skier was telling me about the fun he had on his buddy's lake playing on the kneeboard, and he is in his early 30s.