I've owned/built somewhere in the vicinity of 30 to 40 planes in the past nine years, the majority ARF's, but many built from kits. Not one of them had the center of the wing glassed, and I've NEVER had a wing failure. I've been known to abuse them every once in a while too.

And this has included ARF's where there was only a simply dihedral brace, and the root ribs were epoxied together. No additional bracing to the spars, no carbon fiber, simply built per the instructions.

Maybe that's the key?
Dennis-