Well said 29351; another couple of points - most fuel is burned during the intial climb, so split a single sector in two = two climbs, which may be = nearly twice the fuel (OK so the gtw would be less!)
Likewise, safety, not may aircraft go down during cruise - take off and landing are the crucial phases, increasing these may increase the risk of more accidents. Add to that ATC delays and the wear and tear on the cycles of the airframe, engines, etc...