Hey Fern... I think it is worth owning 100 shares just for the onboard credit - especially if you cruise a lot. You can buy that for about $3500 right now ($2800 for Royal Caribbean) and there is a very good chance they will retain that value or close to it for the next year or so.
I predict they will even go up within two or three years, in which case you get that onboard credit AND will make a profit at the same time.
Now, $3500 is a lot of money for some people - so I wouldn't do this if there is a chance you will need the money soon - because the stock can always go down. But if you already have a stock portfolio in an IRA or 401k there is likely no harm in buying the stocks to hold longterm.
(any stock investment involves risk - a stock can go up or down at any time).