As Mike has stated, the Passport Card can be used for closed loop cruises. And yes, there are a bit more 'user friendly' for identification than a passport.
However, they cost $55 for the first one and $30 for renewal. If you've already paid for a Passport, to me this is redundant and an unnecessary cost.
Plus, as Mike also stated, if you happen to get stuck in a foreign port and have to fly home, you'll need a Passport as the Passport Card is not permitted for international air travel.
Besides, you really only pull your Passport out twice during your travel; once to check-in for your cruise and once when you are leaving your cruise. You don't need to carry it with you when you go into port, so you can just leave it in the safe in your stateroom. And you only need to show your driver's license for your domestic flight.
Note; I do suggest making a copy of the passport and putting that in your backpack that you wear into port. I would also suggest scanning a picture of it and emailing it to yourself. That way if you ever need the information, all you have to do is open your email account.
Basically, after traveling all over the world, carrying a passport is just not a big deal to us, so it's never been a hassle to take them on a cruise. So my advice is save your money and buy a couple more drinks on the ship.
But hey, if you want to spend the extra money just for the convenience of having a card, go for it. It does make it a bit easier.