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Any commercial-software gaps in an otherwise open-source infrastructural framework are a strong motivating force for redevelopment in the public space. Like some force of nature, when a commercial wall exists between two strong pieces of open-source software, there's pressure to bridge that gap with a public solution. This is because every gap can be crossed given enough resources, and if that gap is small enough for your company to cross with your own development team, it's likely to be small enough for a set of motivated developers to also cross.
Let's return to the database example: say you decide to open-source your core SQL server (or your advanced code on top of MySQL), but decide to make money by building a commercial, non-source-available driver for plugging that database into a web server to create dynamic content. You decide the database will be a loss leader for this product, and therefore you'll charge far higher than normal margins on this component.
Since hooking up databases to web servers is a very common and desirable thing, developers will either have to go through you, or find another way to access the database from the web site. Each developer will be motivated by the idea of saving the money they'd otherwise have to pay you. If enough developers pool their resources to make it worth their while, or a single talented individual simply can't pay for the plug-in but still wants to use that database, it's possible you could wake up one morning to find an open-source competitor to your commercial offering, completely eliminating the advantage of having the only solution for that task.
This is a piece of a larger picture: relying upon proprietary source code in strategic places as your way of making money has become a risky business venture. If you can make money by supporting the web server + plug-in + database combination, or by providing an interface to managing that system as a whole, you can protect yourself against these types of surprises.
Not all commercial software has this vulnerability -- it is specifically a characteristic of commercial software that tries to slot itself into a niche directly between two well-established open-source offerings. Putting your commercial offering as an addition to the current set of open-source offerings is a more solid strategy.