Agreed with coconutgroves about the mangroves, tho they do make a great rod for trout fishing from drift boats for the mending properties of it. I think a shorter stick has more advantage with close end shorts into tight mangroves, than with a longer rod. Besides, nice roll casting, even snake casting can be done just fine also with a 9ft rod from a skiff. I'm actually considering going to a softer tip action rod for that, as opposed to my extra fast action rods.
That being said, one of the best situations for a 9.5ft 7-8wt rods is wading the grass flats. The extra length helps hold the flyline up off the water while casting and carrying a lot of line. One of my favorite rods of all time for doing this, like Steve (sjrobin) mentioned was the old school GLoomis IMX 7wt and then that rod in a GLX (but I really loved the IMX the best). Very sweet casting rods for that purpose. Not exactly necessary for boat fishing, tho maybe ok for an SUP or yak. I currently don't own a rod longer than 9ft due to cutting out rods that I don't use very much anymore. I still wade occasionally, but I get away with a fast 9 footer and just shoot more line if needed. Tho it doesn't mean that a longer rod isn't sweet to cast, cause it can be. But again, I don't have that need that often and how many rods does a guy really need?
I really don't see an advantage to beach fishing with a longer rod, since you are mainly on the beach itself (or at least, I am) and casting down the beach to the fish. If you were wading, then I guess it can come in handy and... can be a pleasure to throw, especially in the 7wt (or at least for me).
Ted