Honestly, I'm not quite sure what that means. In terms of world-building? Character development? Serialized arc plots? Entertainment value?
The way I see it, the worst thing a Trek episode can be is boring or forgettable. Whether it "adds to the franchise" doesn't really matter as long as it's fun and engaging in its own right: because it's dramatic, or or heart-breaking, or scary, or funny, or whatever. And, honestly, sometimes even the nuttier episodes, like "Threshold" or "Sub Rosa" or "Genesis," are more memorable and entertaining than any number of staider, less ridiculous eps.
One of the great things about Star Trek, IMO, is that the format is flexible enough to accommodate lots of different kinds of stories: courtroom dramas, topical allegories, morality plays, war stories, spy stories, tragic romances, out-and-out farces, and even, maybe, a spooky Halloween tale.