While the ND does limit the resource, I can understand why he chose the license. I think that, for those who have never released anything under a CC license, it can feel like a daunting and quite intimidating task to give up control of what you create.
I like that Joe included his reasoning - like he felt it was a good first step. I wonder if, as time goes by, he will review his decision as he gets more comfortable with the idea of having an openly licensed resource?
I think it's important that the ND clause doesn't neccesarily mean that the original author is opposed to someone modifying their content. Sometimes it is just a matter of contacting the author and asking if you can make a derivative. I can see that some authors may want to include an ND as a way to make contact with people who are using their resource. I know that Joe is creating a communtiy around his resource. Maybe his strategy with the ND caluse is also to make contact with people to ask them to come into his community? As far as sustainability models go, it seems like a pretty decent strategy.