Well, mine field maintanence is rather difficult when you put your mines on the other guy's side so some concessions have to be made.
At Guantanamo, as I noted, the "Fence" is only a flimsy chain-link fence as you would find between your house and your neighbor's house. The real defense starts with the U.S. mine field on our side as well the Cuban mine field on the Cuban side. Apart from the mine fields, U.S. Marines are stationed in bunkers throughout the area. You don't depend on mines alone.
A mile-wide buffer strip of land is a cheap price to pay for a secure, impassable border.
The thing is, Israel leaving Gaza all together (which this Sharon plan doesn't) will increase the size of Israel's border to defend against the Arab enemy. If Israel has full soverignty over Gaza, it has the sea on the west, Israel on the east and on the north, and it simply has to defend a small border at the south. If it gives it back, it has a much larger border to defend.
