5200 absolutely does offer greater sealant properties - this is the whole point: if it adheres better, it will protects better against water intrusion.
I agree with most of what you are saying, and yes - using 5200 may not be the best approach, or not as good as an epoxy job, it maybe harder to remove etc. That's all true and I believe it.
BUT
If I'm bedding something permanently into fiberglass or wood using sealant, and I want to maximize prevention of water intrusion, I am going to use 5200 rather than anything else (that I know of).
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I think you're mistaking sealant properties for adhesive properties. It's a common mistake; but they're two distinct things.
Yes, the sealant has to adhere to both surfaces in order to effectively seal out water. However, beyond basic adhesion, greater adhesive strength doesn't offer any benefit. Whether you have adhesive sealant with 10psi or 500psi of tensile strength, if the two materials being sealed are held together mechanically and the sealant isn't being subjected to any load, the sealant effect is the same; adhered is adhered. Greater adhesive strength simply makes it more difficult to separate the surfaces in the future if need be.
If, on the other hand, the sealant is serving the dual purpose of preventing water intrusion and holding the two surfaces together, then yes, an adhesive sealant such as 5200 is a good choice. Admittedly this is a gray area, because there are cases in which the adhesive sealant can add strength to a mechanically bonded application. In this case the removability of the fitting needs to be weighed against the possible benefit of greater adhesion.
Even the example of through-hull fittings is somewhat debatable. A properly installed through-hull fitting is mechanically fastened to the hull, either by use of a stem nut or a flanged seacock. The sealant exists to prevent water intrusion around the imperfect mating surfaces of the outer/inner hull. In this case, 5200 is still not a bad choice, as the chances of you having to remove that through-hull fitting are probably pretty low, and the strength of the adhesive on the surfaces of the nut/seacock may help to prevent them from working loose. But from a purely sealant standpoint, the mechanical fastening should be carrying the load and therefore the adhesive is not strictly necessary.
At the end of the day, it's important to remember that a lot of this is really academic in nature. These products all perform a similar task, just with varying properties. Provided the materials you're sealing are compatible with the sealant you're using, the surfaces are prepared according to the manufacturer's instructions, and the proper curing times and conditions are observed... you're going to be just fine regardless of the product you're using. The reverse isn't quite as true; if you need an adhesive you need to choose a product that is designed to perform that function. But a LOT of this topic is the maritime equivalent of Ford vs. Ram vs. Chevy, much like any discussion about electronics, outboards, reels, rods, and beer.
Except for silicone. Don't use silicone.
