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It’s a function of the business requirements. If good is good enough, then open source systems can be valuable. However, if accurateness is a primary concern then the risk certainly out-weighs the reward of saving a few dollars. Businesses have a responsibility to their clients (albeit internal or external) to provide a standard of services that can be relied on. If capability and accurateness is a constant concern, open source may not be for you.
The largest cost of a GIS is people, and the productivity of those people is heavily influenced by the "fit" between the tools and their skills/experience. If the GIS users are not able to use open source tools without significant training and support, the decrease in productivity will more than offset the savings in software licenses.
I'd at to what @David Mosher? said, open source has been a huge benefit to the speed and evolution of technology, but for commercial businesses and regulated industries, the risks of piracy and malfeasance to open source are constant and risk hurting brands associated with embarrassing hacks. Also, limitations to bug fixing and code maintenance carry financial consequences that don’t apply to academia and start-ups seeking new knowledge or delivering a new service or product to market.
Focus less on the richest possible toolset and the lowest possible license cost, and more on the desired business outcomes and total cost of ownership.
Accountability – make sure that the tools you implement have a roadmap, a quality assurance process and an existing eco system of experts (developers + implementers + users).
Time to value – does the tool have the vital GIS functions, the needed capabilities for data access and integration, and the possibility for customization and automation
The topic of business continuity is a fascinating issue because this GIS industry is still relatively small and the generation of those who grew up with GIS are now retiring. Students are challenged by the question of what to study: an industry discipline or to major in GIS. I would discourage majoring in just GIS.
For me this is about business requirements and risk. The business has a need to manage and progress location intelligence in order to grow and gain an advantage. but at the same time as GIS professionals we want to use new technology and developments to push our capabilities. The balance is about managing that risk.