The cruise industry as a whole has been having issues. Numerous cruise lines have built too many ships, and as a result, in the aggregate, there is far more supply than demand. The cruise lines with excess capacity have been offering great deals to passengers (and incentives to travel agents as well, I can only surmise from your question) to get people on their ships.
DCL is the anomoly in that equation. They have only two ships, and those ships are full almost all of the time. As has been stated, they charge far more than other lines (the power of the Disney brand enables this), so their "always full" status is not the result of undercutting prices (trust me, we are in a Category 3 suite on one of the high-demand West Coast cruises--there is no "price slashing" being done).
Book your trip (if you're lucky enough to find a spot) and don't worry. By the way, the DCL has said they are going to make a major announcement in LA in July, and the rumor is that they will be announcing the addition of more ships to their fleet. My bet is that however many ships Disney builds, they will be filled, most likely at the expense of other cruise lines. When we docked in several ports in October, there were several other cruise ships in port at the same time. Almost everyone we met from those ships told us how lucky we were to be on the Magic, and that they wish they could have been on our ship instead of the one they were on (some said they tried, but couldn't get a room).
If the DCL were a separately-listed stock, I would buy it.
Chris