I believe the Me 323 came about when somebody in the LW got the idea to take the original glider version (the Me 321) and slap four low-powered French surplus radials on it. It couldn't take off by itself at full load--it'd still need towplane(s)--but it could fly home on its own power after dropping off its cargo, and admittedly having some power available would make gliding it in much easier.
But in the end it was easier to put six engines on it and make it self-sufficient.