As Charlie P. suggests, there is a good reason to use it, but not in order to glue wing halves more securely. They won't glue together any better with milled glass filler.

Specific to joining two wing halves together, it is unnecessary to use any filler as the joined area is very, very large. And getting a perfect mating of the two root ribs is actually unnecessary. It's good to try for, but not at all required. Most center wing structures do benefit from the perimeter of those ribs being attached directly and that's where epoxy gives excellent performance. So when you're joining wing halves, don't labor over getting the two ribs perfectly covered overall. Just make sure the outer edges are completely painted. And have the ribs around the spars and LE decently painted as well. But it's really unnecessary to work at getting 100% coverage.

One thing to always keep in mind when using epoxy for mostly balsa to balsa joints, is that even if you were to get a perfectly uniform 100% epoxy rib created between the two wing root wing ribs, that bulletproof new epoxy rib would simply be attached to balsa sheeting etc about 1/32" away. And if the joint has ply or aluminum or whatever in it, those materials are there in perfect amounts to provide all the strength the structure can handle, and the epoxy is just bonding agent.

So don't stress over laying in the epoxy. After all, every day people glue those same wings together with elmers and such. And those last excellently well too.