Just killed some Fire Beetles, and they seemed to have "dropped" their bellies... Any particular reason why I should carry them along? Can they be sold elsewhere or are they a required quest item?

One should be plenty.

Some magic items can be created using certain reagents like the "belly", but don't carry a bunch around. They are plentiful.

Actually, I've only found the opportunity to use one once, and that's been in the 2nd chapter and didn't relate to any quest or magic item. Even then, it was just one of many items that could have been used for the same purpose.

Okies, I'll just carry one along for just in case purposes. Thanks for the info! ;)

You can pick them up and sell them for 1GP, but do not bother carrying one of each around the whole game.

Because:
Unlike the weapons components (diamond, ironwood, dragon blood), reagents are available for purchase or in a desk near the laboratories when you need them. So if have even one level in Sorc/Wiz (I have not tried a Bard...), you can use those reagents to create items in Eltonna's lab's (Staff of Power, Gargoyle Boots, Rings of Protection, ...).