Doing this inside a framework makes sense. You don't want the end user
to import a bunch of classes so you instruct him to import just one. That
header will take care of the rest.
Doing something similar inside a project can help you simplify things if
you always import a.h b.h and c.h together, creating a header file called
abAndC.h would also make sense.
As a side note I always import my Constants.h in my pch file to avoid
importing it through my project.