When does root bending moment start to become important? I guess for small aircrafts ,hangliders etc root bending moment is not important at all,because it is relativly easy to achieve sufficient strength ,so their ideal solution will be eliptic..?
Does that mean,small aircrafts dont have any benefits of using bell distribution?
I am also interested what is  root bending moment/wing weight ratio between small aircrafts and big ones,airliners etc?
Root bending isn't an issue for braced structures using wires or struts.  The root fitting is just a pin joint on most wire and strut braced aircraft, and even if there is a one piece beam going across, like some hang gliders, it's not under significant bending, except at the point where the wires/struts attach outboard.
Root bending only matters on cantilever wings, and the structural considerations and calculations will be the same for any cantilever wing aircraft, depending on the design G limits, and the dimensions and loads involved.  It's a beam with a tension member and a compression member and you work out how strong the top and bottom caps need to be as with any cantilever beam whether it's an RV-6 or a 747.