I can’t find UE-17165 in the issues tracker (https://issues.unrealengine.com/).
I think this issue is quite important, simply because they allow you to create complex custom widgets that you can preview in the editor, which makes using UMG a lot easier and more flexible.
Without it, you can only see your generated widgets when actually running the game, which can be a serious slowdown when creating and fine-tuning your UI.