Although workstation-clusters are highly problematic because of their non-scalable communication-behaviour, they pushed the development of commercially available packages for MPPs a lot. The reason for this is the number of installed systems which is orders of magnitude higher than that of real MPPs.
In fact each agglomeration of more than one workstation can be used as a cluster.
In contrary to MPPs, where only a few licenses worldwide can be sold, this gives software-vendors a solid base for the economic success of their porting activities.