I'm sorry you felt you could not do anything useful with the current set of features in Adobe XD. Before releasing publicly, we put out the product to several thousand users under a private prerelease. The tool's vision is to help you to quickly build interfaces, make interactive prototypes and share them publicly in a very short amount of time. In talking to our private prerelease and gathering usage data, we found that we had hit a point where the tool was delivering this value to some of our users. In addition, we had gotten to a point where our internal designers have been using Adobe XD to design XD. As a product manager, I've also found value in XD to quickly mock up flows that I previously did in Keynote, but, find much faster to create now.
So, to circle back to your question, "Why would I ever use an incomplete product Adobe?", the answer is because it might provide you some value in making your workflow more reliable and efficient. If it doesn't meet that need, then you can choose to move on or be a part of a community helping to shape the future of a tool you would love to use on a daily basis, maybe not now, but, later.
Lastly, scrollable content... I want it too. It is coming soon, our eng team is working on it. But, honestly, one of the reasons we have prioritized this amongst a VERY long list of features is because users have upvoted it so much on our feedback site. We need the feedback to build this tool and are extremely grateful to those who are providing it. In return, we hope we are providing them value too.
Without scrollable content you can't create responsive prototypes on Mobile, this is a must for almost anyone other then software designers. From a business prospective why would you release something not ready for primetime? This approach obviously didn't work with Edge Reflow so why do it again?? It only drives users like myself to competitors (Flinto, Sketch) never to return. I understand why Adobe is doing this I just don't agree with it, and as a long time user and designer I feel this crowdsourcing approach not only looks bad from the outside but it will most likely kill a potentially great product before it ever has a chance.