One advantage of disengagement is that it diverts world attention from continued building of the fence in the West Bank. I happen to agree with David Makovsky that the fence should include about 15% of the West Bank, leaving about 85%, give or take a few land swaps, for the final agreement.
The Palestinians were offered a phenomenal deal at Taba, which included 97% of the WB (close to 100% including swaps), all of Gaza, the Arab neighborhoods in Jerusalem, horizontal sovereignty over the Temple Mount (actually Israel offered complete sovereignty with recognition that the area is holy to Jews, which the Palestinian rejected), and a family reunification plan. After a violent rejection that lead to the death of thousands, they cannot expect to get the same offer again.
So leaving Gaza lets us keep more of the West Bank. Isn't that itself a good enough reason to withdraw?