There are plenty of reasons one would choose a smaller, human powered vessel for the sound.
But personal preference aside, and limitations such as storage/towing etc, I can't think of anything in Puget Sound that can be done with such a vessel that can't be done with a 15' aluminum boat. There are, however, many things that the boat can do that those smaller vessels simply cannot.
Just my opinion, but unless you're just drawn to a small human powered option for whatever reason (and no doubt there is something to be said for the simplicity and likely intimacy of, say, a kayak), and storage/towing/maintenance isn't a concern, I just dont see a scenario where I would choose one of those smaller options.
I definitely can't launch my 19' boat at many places where you could throw in a kayak or a SUP, but I can one hundred percent fish all of the same water and soooo much more. It also costs more to own, operate, launch and maintain a boat, so no doubt that is a factor. But for me, the complete freedom that a motor boat provides on the sound is something that truly can't be matched.
Maybe if you have the option and could borrow a small craft from a friend or something, give it a shot and see what you think. Also, if you have an opportunity to hop on with someone who owns a boat (and don't have experience fly fishing the sound from a boat), give that a try as well. Either way you go you'd be spending a decent chunk of change, so no doubt you want to make sure you make the best decision for your needs.