Putting aside questions of whether or not a fence should be built, does anyone defend the path that Israel has chosen for this fence?
http://www.jr.co.il/pictures/israel/maps/map0307.jpg
As this map shows, the completed fence and projected plans for the fence extend far past the Green Line of 1949.
It seems obvious that Israel is expanding its borders by simply placing large swathes of land on their side of the fence. The only defense I can see is that it is necessary to do this to defend the settlements in these areas. However those settlements are illegal and many are supposed to be in the process of being dismantled.
In short, I think Israel would be justified in building this wall if they respected the 1949 boundaries. As it is, I cannot see how one can defend the construction of this particular fence.