I do (currently) sail only one cruise line and the stated, and unstated, loyalty benefits are a major reason. I like the cruise discounts, shipboard credit, free internet, free phone, pressing, newspapers that I currently get and free laundry and air deviations (and free phone time increases to nine hours per trip) that I qualify for on my next cruise. I also like the additional "benefits" I gain just from being known by the officers and crew.
So far, I have not run out of itineraries. Yesterday, I booked a Monte Carlo to Rio crossing for November 2012; only two repeat ports out of ten. If I do run out of itineraries, I will have to search a little wider for cruises (I am taking Silversea to Antarctica, for example).