Okay from my little experience about planes and living nesr an airfield, i can say that you get (almost) twice as much lift for a given wingspan. It was difficult to create long wings that were stiff enough to support themselves and the aerodynamic forces. Biplanes not only give you shorter wings but you can easily brace them against each other. The problem as you pointed out is that they are complex and give lots of drag. Some are used today, tandem wings (with one wing in front of the other) or canards (small wing near the nose) - they are more manouverable and usually don't need horizontal tails.