I glass the joint if it is a kit, but not for an ARF. I had a GP Super Sportster .60 that was not designed to use a joiner on the spars, and relied on the wide (4 -6" wide?) 6 ounce glass joint for all its strength. It worked great. Other kits use relatively small wing joiners glued to front and back of spars, then add a 1" wide glass tape over the joint. Most ARFs are designed knowing that the majority of assemblers are NOT going to peel back the covering and glass the joint, so they are designed with long and strong joiners (OK, the good ones are, anyway!). Keep in mind that it takes a bit of work to fill and sand the area after glassing for a smooth surface to re-cover, and lots of ARF builders don't have covering tools.

Bandit, If it makes you feel more secure- do it. The worst that could happen is you add a little bit of weight to your plane.