Yes I have used them, and yes they have their place. In my hull, I have a fully accessible wiring chase, by way of a 6 inch wide piece of deck that is screwed on. I have rivnuts for #10 machine screws into the structure below, and then my deck is held on with #10 Oval head screws, through finishing washers.
I don't see, however, the use of them as a solution for dissimilar metals. The only way to eliminate the issue of dissimilar metals is by the use of nonmetalic bushings, as well as a nonmetalic isolator pads (basically rubber between 2 metals, and a bushing around whatever fastener holds it on. Unless you get plastic rivnuts, you still have dissimilar metals. Say your deck is aluminum, and the rivnut is aluminum. Being that both are aluminum, they will be similar. (Yes, different alloys technically are dissimilar, but to a very minor extent that dissimilar corrosion doesn't take place except in the absolute harshest conditions - much worse than any recreational boat will ever see). Now, if you try to use a steel bolt, you still have dissimilar corrosion, but instead of it being between the deck and the bolt, it will be between the rivnut and the bolt.