It needs to work as a listening experience that expresses the story arc.
This could be an interesting discussion point. I think we'd all agree that this is what we'd most like a cast recording to be, but are they in general? Having a synopsis helps us fill in the gaps that we don't hear, of course, but taken just as a listening experience, I don't think that we always get a true sense of story arc on albums like this. I think it depends on the show itself a great deal, and how the music itself tells the story (obviously it's much easier to do with a show that's through-sung or mostly so).
Certainly I don't think that early cast albums strove to do much more than to present the songs - and in those cases, a printed synopsis certainly did help. And in this case, yes, even though there is a synopsis online, I get the idea that if you're going to buy the physical CD it's nicer to have that there as part as the complete CD package. But I would wonder whether the album producers were really thinking that they needed to give us that kind of aural storytelling experience that you're expecting, or if they were just trying to document the score. Given the craziness of the show's plot, I agree that a synopsis would seem very appropriate. But maybe they were thinking along different lines?
I trust we'll all have various opinions about this. ;-)