Here are some of the arms I was referring to:

http://www.campbellcameras.com/p-140-dfo-tree-arm.aspx

http://www.huntercamcradle.com/hcc_pro.htm

Now that I look back through many of the other arms I've looked at I see they have one strap too. I guess it is more in the design of the base that allows the strap to provide stability.