One of the goals of the PREP workshop is to develop problems and sets that are pedagogically sound. Accordingly, I would definitely expect that the materials produced there would use solutions and hints in an effort to reach that goal.
I should add that I think there are many very good problems in the NPL that do not include hints and solutions, but that as a rule I expect that inclusion of these may improve the effectiveness of the problems at encouraging student learning. I believe that one of the shortcomings of some sections of the current NPL is that problems do not uniformly have full solutions (etc.), but I do hope that we'll be able to help address that.