One of the demanding feature for Windows 10, both for Phones and Desktop, was an easier way to side load apps. There are various reason why one will want to get it done which includes beta testing to just use an old version of the app. Till date, Microsoft had made it little tough for anybody to do it. You either need to register or run something for this to work.
Looks like Microsoft is finally accepting the fact that this option is much needed, specially with more Android and iOS Apps on its way, this is a necessary option which Microsoft needs to open.
Daniel Rubino from Windows Central cited this very option in one of the sessions in a Windows 10 Build, which works now. He has got it confirmed that all Windows 10 devices, including phones, tablets, and PCs, can be developer unlocked through a toggle switch in Settings.
However, Microsoft has put up a limit to install 20 side loaded apps or games for phones, but on Desktop it has been set to unlimited.This feature however can be locked down on Corporate phones, by admins, to limit unsigned apps get into the company.
Its good to see company getting along with developers and consumers little better. Lets wait for this to rollout in the next preview.