Tim Mertinooke wrote:..possible melting concern if subjected to the heat of a lightning strike.
"possible" being the most telling part of this boater's myth. I can't quite figure out why people would feel this would be true. Frankly, if one had a lightning strike that was truly inclined to damage seacocks in any way, it would likely affect both metal and Marelon fittings in bad ways. In my opinion, this is not a reason to choose one over the other, regardless of which way you might lean.
Marelon is nylon fiber-reinforced plastic--rather like your boat, in fact, which is also fiber-reinforced plastic. The term "plastic" conjures up all sorts of cheesy images, of course, but the reality is that Marelon is really an engineered fiberglass product (that uses nylon instead of glass fibers), and, however much it falls under the basic definition of a plastic, it's really not, at least not in the commonplace, toy-section-at-Wal-Mart sense. It is strong and long-lasting. I am sure some very basic maintenance is required to prevent the balls from sticking--just like with any such fitting, metal or otherwise.
Tim Mertinooke wrote:...stories of handles breaking, cracks forming...
You can find stories of failures for any single product or material out there in the world. Just because it happened to someone doesn't mean it's common, or even remotely likely. Everyone loves a horror story, it seems.
I originally bought Marelon fittings for my boat, but returned them when I realized that the 1-1/2" fittings were huge and too large for where I wanted them under my cockpit (so are most bronze seacocks of the same thread size). Marelon didn't have a 1-1/4" fitting (which I used in bronze along with full-flow tailpieces for 1-1/2" hose), so I switched to the Conbracos.
The Conbracos are sound fittings. I hate the handles, but they work; replacement of the handle is a cinch if something happens to it. All my handles are still in fine condition. With a tiny bit of what I think should be normal overall boat maintenance (read: not neglect), the handles don't simply rust away and disappear. I'd rather have cast bronze handles, obviously, and would seek such a thing on my next installation.
I still prefer bronze, but Marelon is a good product, I think, and the more Chinese the bronze products become as manufacturers seek higher profits at the expense of the consumer, the less advantage bronze has, if it has any to begin with. I wouldn't hesitate to use Marelon if it fit the application, and think that it's earned itself a solid track record.
Neglect kills everything. Take care of your boat--all parts of her. Especially the ones you don't see all the time.