Leaked Windows builds are nothing new. What is new is Microsoft's stance on the use of Build 9901.
"How could we be upset about lots of people wanting to try our new stuff" Gabriel Aul, an engineering general manager at Microsoft and point person for the Windows Insider program, tweeted after being asked about Build 9901. "We'd prefer you stick to official builds though."
Microsoft hasn't always been so ambivalent. Just this past May, for instance, former Microsoft employee Alex Kibkalo was arrested for leaking Windows RT prior to that operating system's launch. (Kibkalo's leaking of activation server code probably irked Microsoft more, however.) Microsoft was also said to have fired two employees for leaking Windows 8 ahead of its launch, though Microsoft never officially commented on the matter.
Happy harmonious times aside, users who decide to seek out and give Build 9901 a whirl will still need to install the official Windows 10 Preview build anew from ISO the next time an update rolls around--presumably in late January. A bug in the leaked build removes the ability to receive Windows Insider updates, though "not punatively," Aul writes.