To me, as soon as you have more than one thing in a map, you need to let people know what's what, and to that end a legend is useful. I see this as a general thing/rule and am very surprised how/why it is even a consideration NOT to have legends. I was extremely surprised and disappointed when I first realized how limited the availability of legends are in StoryMaps.
For my current specific example: I work at a Land Trust and want to show the public our conserved lands and I intend to use the Explorer map to do this because it allows users to 'browse' through our lands back and forwards between the map and list of properties. There are different levels of public access to the different lands and we use a thematic layer to show different categories in different colours on the map. We can write somewhere on the page that 'This colour = that" but that's really not very intuitive or as visible as a simple Legend.
As is possible in for example an Express map I would suggest this same option is allowed in Explorer maps.
