The statement has been oft repeated on the Microsoft sites. But, Kyle, I think there can be misunderstanding about leaked "builds"
They are not neccessarily leaked from Microsoft sources. Builds, as indeed quoted, are sent out, often to complaining hardware or software developers. In the former case, and, given the circumstances, sometimes to the software people, the source code is included.
As such, it is not difficult for mischevious persons, within those multiudinous organisations, to alter the code. If they are prepared to leak, why not prepared to do worse. I have not read of such instances (using the term "malicious")
However, maybe MS should not, in this particular bulletin, have used the term "malicious". I am sure they meant something milder, such as erratic code which could blow certain aspects of the OS.
No one in MS is suggesting the public should be using "builds", for the reasons stated. They are, after all, for the most part, an adaption from a previous build, or even a public release, and are not, at that stage, representative of the final product.
I can see, in retrospect, it makes the whole point, in forums such as this, of offering help for strange OS behaviour, rather irrelevant. But as an active participant, I beieve it leads to a possible "leaked" (lol) path to feedback for the information of Microsoft, and a cleaner finished product.