I think he had to turn it to save it from Widmore and the outside world.
The reasoning is Jacob, via Christian, told Locke he had to move the island to save it. As far as we know, the "how" to do it was known only to Ben. So Ben takes them to the Orchid, knows it's a one-way-trip for the person that turns it, and does it. For some reason (Dharmacaust?) Jacob isn't happy with Ben anymore. Ben even says so as he begins to turn the wheel.
The current premise is that getting the O-6 back to the island will fix everything. Seems to me if somebody went down and turned the wheel back a couple notches, that might work too. Time will tell.