I wonder. RaD clearly relies upon the other novels in The Fall to complete it. I don't think anyone here will deny that assertion? But do you think the other novels of The Fall will depend upon RaD for setup? I have a feeling this won't be the case. They will mention the one or two events from RaD, and then move on with their own story arcs. If such a situation were to occur, will it still be possible to defend RaD? If such a situation were to occur, why would RaD need to be published at all?
Only indirectly. You need to know about the new station, the names of the ancient Bajora, and the big event that took place (see, no gleeful spoiler there!), but there's really no development or plotting you need to have sat through, I think you could safely skip it.
You just need:
-'event' that happened at the station, and that there's some shady setup/missing facts about what really happened.
-there's a prophet/orb thing going on, and that's the backstory for the bit at the very end of book 1 (and Kira is tied up in it)
If you were told those two things, couldn't you really skip this book completely? And if that's all you need to know, was it that good of a book? What am I leaving out that you would have missed? Did anyone grow/change/anything else that would leave them in a different spot than they were when we picked the story up?
While tempting to list things like "Bashir and Ezri make up a little", did it really matter? It was a couple pages, and had ZERO bearing on what happened. If it turns out to be important later, could have easily been written into THAT scene as well without lessening the impact.
Given that criteria, can't really give this book a good grade.
By contrast, you NEED ESB for ROTJ to make sense. Luke goes from farmboy war hero to calm Jedi knight with some control of his powers, there's an obvious progression for the character. Doesn't resolve all of the plots/cliffhanger if you quit at the end of ESB, but you can get a story there with it alone, and it doesn't make sense if you skip from ANH to ROTJ, there was obvious growth/change. Not so with this book. Just knowing the list above makes it skippable, and you won't feel lost at all.