They aren't nearly as squishy as many of the WARs and min/maxers would have you believe. DRKs are below PLD and WAR in some areas, and above one or the other in others. They're basically right in the middle. But from a development standpoint it's much better to start a class off lower than higher, because then if you buff them, people are happy. People always respond negatively to nerfs.

However, they're still very viable tanks.