Boat shoes had a specific purpose, or specific application when they were popular in the Preppy hayday of the 1980s. Sailors and yachties needed non-marking rubber soles that had slits or channels in the soles that provided some traction on a wet surface. Thus, the white soled Sperry topsider.
In the last 20 years, new materials have been introduced into sailing shoes (and gear) so most sailors and yachties wear something that looks like an athletic shoe now. Or, they wear Keens, those ugly but functional sandals with the hard rubber toe covering that were designed by a sailor who was tired of jamming his toe on the rail of his sailboat.
If form follows function, there is no longer a need for boat shoes to look like the original Sperry topsider. I suppose some would buy them because they are nostalgic. Perhaps buying Sperry topsiders (or their look alikes) is similar to buying a Barbour waxed cotton jacket--it has a certain look, but it is heavier and less functional than a gore-tex jacket for rain protection.
To me, the boat shoes looked best when they were weathered: sun bleached and sewater stained. In that case, they looked like actual boat shoes rather than street shoes patterned after boat shoes.
Where I grew up, sailing was a summer sport. Therefore, boat shoes were a summer-only clothing item, like shorts and polo shirts. Also, I looked at boat shoes as being pertinent to sailing--an athletic shoe of sorts. Accordingly, I would not wear them in the colder months and I would not wear them when not on a boat or near water, just as I would not wear hiking boots or ski gear or tennis shoes in inappropriate situations.
In summary, boat shoes have their purpose and in the boating context, they are fine.