Use the right tool for the job. This is one of the guiding principles of good craftsmanship, and it would be a fine slogan for an Open Source and Proprietary Geospatial Foundation.
GIS, as practiced, is more craft than science, and requires both a diverse tool kit and a pragmatic sensibility. Almost every open source-using practitioner, myself included, applies a combination of open source and proprietary tools to their problems. However, the pragmatism that gets us through our daily tasks is not an appropriate ideal for the entity that calls itself the geospatial open source leader. OSGeo should be single-mindedly promoting an end-to-end open source GIS software stack. Interoperability with proprietary software is a means to this end, but is not, by itself, a great enough goal for OSGeo.