Some wings , like a Stafford kit, use a stressed skin design. There is no internal hardwood structure. The strength of the wing is from the skin in tension on the bottom and in compression on the top. To insure continuity the center must be glassed.

Some wings, like the Goldberg Chipmunk for example, are built like a Loiusville Slugger, lots of hardwood and a reinforced center section. Glassing adds very little strength because the wing skin is not being used to carry the load.

ARF's are another animal. The kit must be designed to fit in a box for shipping. The builder usually slides in a plywood brace and glues the wing halves together. Lord only knows what is in that wing. Are you gluing plywood to plywood or are you gluing plywood to balsa ? The ARF may indeed be designed as a stressed skin wing. When in doubt, glassing certainly can't hurt. And if there is a foam core, then you know it is a stressed skin design and must be glassed.

Glassing the center can't hurt. It may or may not be necessary.