The non-commercial provision obfuscates intent
The purpose of a general public license is to establish agreement between two parties, with enough clarity that adjudicating any later disagreements is trivial. The "non-commercial" provision (NC) available with the Creative Commons suite of licenses, however, introduces uncertainty into an otherwise straightforward agreement. The non-commercial provision in many cases should be avoided on that basis alone.
The general purpose of a general license
The purpose of a copyright license is to establish agreement between a copyright holder and somebody reusing their work. The license accomplishes this by employing precise legal language, often carefully tailored, in consultation with a lawyer, to the work, the kind of use, and/or a specific legal jurisdiction. A license is typically a legal document that must be agreed to and signed by both parties.
A "general public license" further eases the process of establishing agreement. A general public license (including all the Creative Commons licenses) consists of boilerplate text that anybody may use, in place of hiring lawyers to craft something specific to their own use. The ability to refer to a general public license by a short name (such as "CC BY 4.0"), and/or link to it with a stable URL, further eases the process of establishing agreement. The copyright holder publicly offers up their consent to a specific general public license; anybody may take them up on that offer, at any time.
"Non-commercial" is non-specific
The distinction between commercial and non-commercial use may seem clear-cut. But consider these scenarios:
- A for-profit company uses a photo as part of a charity drive
- A non-profit university uses a song in an advertisement for a service it sells
- A for-profit company uses a song as the hold music on its telephone system (and does not charge customers for using its phone service)
Are such uses commercial or non-commercial? Reasonable people could disagree; the only way to figure it out is a conversation between the copyright holder and the person who wants to reuse the work. But now, we're back at the beginning; the whole purpose of using a general public license was to avoid the need for one-on-one consultation. That purpose is undermined if the license necessitates direct consultation.
The role of Creative Commons
Creative Commons, the organization, has paid attention to this issue over the years. In 2009 they published the results of a study which sought to determine the public's understanding of the term "non-commercial."  The license text itself attempts to bring some clarity, with the clause "NonCommercial means not primarily intended for or directed towards commercial advantage or monetary compensation."
But if the two parties initially seeking agreement need to bring in a third party (Creative Commons) to inform their contract, the situation has again become more complex, not simpler. And phrases like "primarily intended" may require yet another party -- a court of law -- to weigh in before true understanding is reached.
Limitations of this argument
While there is a substantial "grey area" in which it's difficult to determine whether or not a use is commercial, there are certainly cases where it's clear that a certain kind of use would be commercial (and thereby prohibited by the NC clause), and also cases where it's clear that another kind of use would not be commercial (and thereby permitted under the NC clause). For certain works, where a substantial portion of the likely reuses would fall outside the "grey area," the argument presented in this essay may be less relevant. In such cases, it would be worthwhile to consult other arguments for or against the use of the NC clause. (See the "see also" section below.)
Furthermore, it's possible to expand on the foundation of a general public license; for instance, a website could state "All content is available under a CC BY-NC license, subject to our definition of "commercial use". It might take a bit of extra work, but there is no reason a copyright holder couldn't take it upon themselves to fill in the gap left by the license itself.
A general public license is useful only insofar as it captures concepts well enough to establish clear agreement between parties. If a copyright holder wants to restrict reuse based on the purpose, they are choosing to enter an area that is necessarily complex, and fraught with potential misunderstandings. A copyright holder is not well served by a general public license that only seems to express their intent, but that loses its clarity as soon as its provisions are held up to scrutiny. A conscientious reuser, when faced with a NC license, must contact the copyright holder to gain clarity about the meaning of "non-commercial"; the situation would have been clearer if the copyright holder had simply declined to offer any public license at all, and publicly expressed upfront that the reuser should simply contact them to work out licensing terms.
A general public license that includes the "non-commercial" provision fails to introduce clarity. It introduces needless complexity, rather than reducing the friction of inherent in one-on-one license negotiations. It's better to offer no general public license at all, than to offer one that introduces new potential disagreements.
The essay above highlights only one reason to question the value of the non-commercial option in the Creative Commons suite. Below are some related arguments, as well as some examples of users who have found value using the NC option.
- Licenses/NC presents other arguments against using the NC provision.
- Free knowledge based on Creative Commons licenses, translated and adapted from a work originally by Wikimedia Germany, goes into great depth on the topic.
- Filmmaker is a page on the Creative Commons wiki, with case studies of filmmakers who have used various CC licenses.