I agree with Rolf. If the package development tools (Workbench, MUnit and the profiler) were free, that would improve the quality of packages (most popular packages are free). If there are more high quality packages that would improve both the reputation and the usefulness of Mathematica---in the end probably a bigger win for WRI than any income from Workbench sales ...
Mathematica is a platform and good developer tools are essential for the success of a platform.
I know from first hand experience that MUnit, which is part of Workbench, was essential for the development of MATLink
. Without it MATLink would still exist but it would be much buggier.