If you think about it, the glider is mostly made of a wood frame surrounded by wood, and cloth. It could be used as a "Stealth Plane" because it wouldn't show up on radar. The glider could be useful because it wouldn't show up on radar, and you could sneak into an enemy base and capture it.
Up at 15k in your C47 with two other guys towing gliders and down low at 3k you have like 5 guys with C47s and no gliders and the guys at 15k release their gliders and the gliders fly on a different course then come from behind, land near the town, as the other 8 C47s get owned, and gliders land and drop there troops, because they didn't show up on radar and the base becomes yours.
Yes the C47 is faster and a better choice, but the glider can be used for "Stealth Missions" or used to supply maybe GVs on the ground as the C47 flys to another base to capture it, or to supply other GVs in a different area. I can see some use in the gliders. Maybe make it, if you land close to one of your guys in a GV and you hit tower, you land succesfully and you get some perks.