Ken, there are 4 reasons why a bipe made sense to me.
1. More wing area/ lighter wing loading. If I was to do the same by just making a single wing larger the airplane would suffer in roll performance especially in snap rolls.
2. The two wings help straighten the prop wash prior to reaching the tail. This helps with tracking.
3.Higher drag profile. Drag at the right location is good IMO. It helps with keeping constant speed and helps with tracking as well. The downside is you need to make sure you have the power to overcome the drag.
4. I just think they look BADASS.......
I'm hopeful that some more experienced higher level patter pilots will give more information but those were what I am thinking as I go through the construction of mine.
Yes weight is a big concern however the wings are smaller, much thinner because they don't need to be quite as strong. When assembled the wings form a box structure and gain strength. As I said in my build thread I think we overbuild our models anyways. A big eye opener for me was picking up a buddies CK Alure, very light construction.