For me, this topic is analogous to no alcohol being allowed in the MK.
On the one hand, I sort of understand the concern. Disney is all about the wholesome family experience, and blackjack and slot machines don't really fit into that construct.
But, on the other, it amazes me that Disney, never one to turn down a buck, (goodness knows

) lets this potential cash cow slip through it's corporate fingers.
Personally, I'd welcome a casino on the ship. And this comes from someone that only plays an occasional slot. I don't really think it would attract 'the wrong element', on the contrary I think it's kind of festive just walking through an area with lots of flashing lights and computerized music.
Alcohol hasn't destroyed the family experience at all the other parks outside of MK, and IMO gambling wouldn't hurt the feel of any of the cruise ships.