Aaron -- Most of the action on Monomoy this year has not been sight fishing in the traditional sense.... more like standing on the edge of a deep trench and blind casting to cruising bass. The limited sight fishing that has taken place - at least from what I've seen and heard - is not the typical Carribean style flats fishing where fish are tailing in a foot of water... more like you see a cow cruising the edge of a deep run and with the understanding that you have one shot and one shot only you hurl an intermediate line down to her in reasonably short order, sometimes twitching that crab pattern 'til she's licking your waders
On the South Island trip, we will be on the edge of a very deep, very fast moving body of water and a floating line will be of little use unless the fish are onto poppers.
I do see your point about floating lines in general. I personally haven't gone that route simply because Monomoy alone is not enough to justify the expense of a thrid line. I own a 7 weight with floating line that I've used for schoolies on surface flies (lots of fun), but so far, I haven't had the cajones to try this on the flats.