Why core? The simple answer as suggested above is weight, weight and weight.
It is certainly true that a cored hull or deck
is stiffer than a non-cored deck of the same weight, but that only tells part of the story. A solid hull of the same THICKNESS is a lot stiffer than it's cored counterpart, but weighs a lot more.
To say cored hulls are "stronger" is not quite correct, since "stonger" doesn't really have a technical definition. They are, for example, much less resistant to puncture in an impact.
Above this line is simple technical knowledge, below is my opinion..
If you have a boat that requires the light weight like a hot shot racer
or a cat, then you need the cored hull. Cored decks are a good idea on any boat, they aren't really subject to the kind of impacts that hulls are. If you add the extra weight and build an appropriate solid hull that is stiff enough to hold it's shape in a seaway you will end up with a much more robust boat, albeit a heavier and somewhat slower one.
I think a cruising boat should be as robust as possible, others might disagree.
However, replacing PART of a core with solid glass is a bad idea, The different parts
of the deck will have different flex properties and it will crack at the juncture. If you need to replace core, replace it, don't fill it with solid glass.