Lou, what you say makes sense and you might be right about a product's lifespan not being effected by how long it's left on. I'm certianly not an expert but I was told by the owner of a small charger company that leaving it on all the time would effect the lifespan of the transisters in his units. His reccomendation was to use timers.

In the end it probably isn't that big a deal. The biggest risk I think is lightning causing a dramatic surge in current and possibly frying-out your boat's electrical system.