Easy to use: It has no exposure setting, no focus setting, the flash either fires or does not. That eliminates the possibility of screwing it up; all you need to work on is good buoyancy and good composition, skills that are vital on any UW camera system.
Cheap to use: The entire system is less than $300 and a flood will only cost you a cheap fuji aps camera and a roll of film. Contrast that with the price of just about any lens for the Nikonos. The Reefmaster camera might be fairly close on price while allowing the use of 35mm film, but it still costs a bit more with the strobe kit.
Macro option: with the macro kit, you can get good pictures on your first roll. You need only frame the target and click on the image that is 8" from the lens. The print in 4x6 size is roughly lifesized. Note that you don't even need the strobe to do macro, so the kit could run you under $200. (I personally feel that would be limiting though)
Strobe: The external strobe option is far better than available on many of the cheaper UW kits. Cameras like the MX10 have it far too close to the lens to eliminate backscatter. And though awkward, you can unscrew the strobe arm from the housing and hand position for confined subjects.
Downsides: with simplicity comes limitations. It's fixed focus lens works well for photographing items from the size of a basketball to the size of a sea lion, but it doesn't do wide angle well. The macro is good for items the size of a tennis ball, but falls short when it comes to itty bitty creatures like nudibranchs or other individuals the size of a dime.
With the nikonos extension tubes, you can take an image that is 1:1 with the negative. Eben took these two images of a single strawberry anemonie and section of a spiny sea star on his first roll. So, you have to accept that there are certain subjects your aquashot is well suited for, and some that just are out of its league.