The theory behind this is that the students will learn valuable lessons about finding sources such as how to use a library or vetting for reliability. So what's the problem? Recent studies "tell us that students can find sources; the trouble is they don’t read them, or they read only enough to find a useful quote, or they choose sources that are not particularly insightful ones, or their paper becomes merely a description of the sources they’ve found with little analysis or original thought. A more sophisticated mistake is to seek out only sources that support a previously-held belief."
A preprint of an article proposes a solution: "stop talking about 'finding sources.' Frame the work as learning about something. When sources are viewed as containers, it potentially diverts attention away from the content of the sources themselves. Likewise, a discourse of 'learning about' directs attention to the content of sources. If internalized, both of these conceptions might serve as psychological tools that mediate how students view and engage in the research process."
This process is more ambiguous than simply finding sources. The student will have to use research strategy to learn about the issue, then continuously revise their research to fill in holes.
"First 'I want to find out about X,' which requires wide but shallow exploration. Then 'The question I have about X is . . .' which requires a more focused search. And finally 'I think . . . about X, and here’s why,' which means organizing ideas, coming to conclusions, and drawing on the evidence available (which may require further searching if a hole appears)."
It wasn't until I read this article that I realized I was inadvertently using the more ambiguous, 'learn something' method of research in my Scholarly Writing class. I don't require any set number of specific resources, but we do discuss reliability and how the use of various resources can make a paper stronger. For example, I expect my students to do novel analysis of primary legal authority instead of merely relying on secondary sources. But the use of secondary sources is appropriate to substantiate an argument, especially for a law student who is not seen as an expert in the field.
Most of the law students are comfortable with this practice, and it generally eliminates the misuse of resources just to meet a quota -- like pulling a quote out of context. In the higher education writing and library worlds, we must be cognizant of this issue so that our students are fully equipped to find and analyze a source. It's not enough to find a book, pull a quote, and call it good.