Hadley - i *think* it still holds true for Moissanite. because, while it does have a funky cut... it is a funky cut that is at least somewhat optimized for the material, if that makes sense?
so, really, i think it's just poorly cut stones that might benefit from getting as much light in as possible.
i like warmer coloured diamonds, and don't mind visible inclusions, so i don't mean this as an insult to those kinds of diamonds :P
but for warm, poorly cut, included diamonds? setting them in closed in settings will make it obvious how badly cut they are. so that's why - i think - so many maul jewelers will say things like 'see how open the setting is, to let in so much light so your stone will sparkle!'
... because those kinds of stones need all the help they can get, to look whiter.
and you might see twinkly reflections from light hitting the outside portions of your stone sometimes, too, in reflections.