Hi John. Gotta disagree when you are talking about real world, enterprise, warfighting functions. If you want to collaborate, share, and improve SA for decision making in JIIM environments the ArcGIS platform with industry standards, formats, support to open standards etc. IS the way to go. Our partners in US Gov't, NATO, FVEY etc. usually have ELAs to utilize ArcGIS. This allows the common understanding of REST services, use of extensible APIs and widgets and more. NATO has just renewed their ELA and announced they will be utilizing it for more than GIS- it will be the foundation of their C4ISR technology.
I would agree with Todd though- Is there a part of your job function related to creating and sharing geographic content? What do you need from a GIS? What information products do you need to create, share, archive etc.