At the moment there isn't a clear statement on the help page "How do I write a good answer?" saying that answers should preferably include references to back them up. For "good answers" I would say that this is almost a must.
To me, it feels like we have had an influx of new users (good) that sometimes post answers based on loose assumptions, often structured as an open discussion/argument, without any kind of references or background information to back them up (bad). These users are then often referred to the page "How do I write a good answer?". It is therefore unfortunate that a statement about the value of references is not included there.
The help page on How do I ask a good question? does include a statement on references, and something similar would be useful on the good answer page:
When in doubt, ask people to cite their sources, or to explain how/where they learned something. Even if we don’t agree with you, or tell you exactly what you wanted to hear, remember: we’re just trying to help.
A suggestion from me for the "good answer page" would be (maybe too specific though):
Try to include references to back up the claims in your answer. Even if not absolutely necessary, it will strengthen the answer and can provide a point of entry into the literature for users reading it. It will also make it easier to evaluate how trustworthy your answer is. The most reliable references are research papers and textbooks, but also other sources such as science blogs and wikipedia can be used.