This is better described as "niche" expertise than anything significantly 'deep' or 'high-end'. Basically, stuff that (1) is not truly business-critical, at le

This is better described as "niche" expertise than anything significantly 'deep' or 'high-end'. Basically, stuff that (1) is not truly business-critical, at least compared to the closest FLOSS equivalent, and (2) is so inherently niche that it would not derive any benefit from the typical network effects that drive people to standardize on open source solutions. ESR himself discusses this case in his book The Magic Cauldron; one example he provides of something where FLOSS is probably not called for, is software for calculating cutting patterns for sawmills. A different one, is the business-management "secret sauce" that's embedded in many ERP packages. Note that the "not truly business-critical" proviso is quite relevant; for something that is critical to your business, however "niche" it might be, not having access to the source code would be an unacceptable risk!