That's not a stupid question, but it's difficult to answer.
Fiberglass cored with balsa makes it lighter, and stronger, and can also help reduce the transmission of noise. That's good! That's why so many (nearly all) mfr's use some sort of coring in decks. Fiberglass would have to be too thick and heavy to be strong enough on it's own.
In the 90's, SeaRay and other builders started using more and more balsa for the reasons stated above. The bad part is when the FG skin is penetrated and water is allowed to compromise the integrity of the coring. (deck fittings, through-hulls, screws, damage, vents, windlasses etc) There was a belief that "end grain" balsa, even when subjected to moisture, wouldn't wick the moisture deep into the structure. Turns out, it does, and one could argue that SeaRay wasn't as careful as they should have been about protecting such large areas of below-the-waterline wood.
We're at a point where these boats are old enough that surveyors tend to go DIRECTLY to the hulls with moisture meters. Buyers have to be careful about moisture content in potential boats, and owners have to make sure their cores aren't subjected to moisture.