In the absence of Berserk and IR, Fell Cleave has an efficiency of 7 potency/gauge if it replaces HS, 5.2 potency/gauge if it replaces Maim, and 3.58 potency/gauge if it replaces SP. Details are here. Compare that to Onslaught's 5 potency/gauge and Upheaval's 15 potency/gauge. Fell Cleave's primary advantage is the lack of a recast cooldown.

Under IR/zerk, you can also show that the only GCD in which FC is better is when it replaces HS. If you replace Maim or SP in the window, Onslaught yields a higher potency efficiency.

I'm not sure under what circumstances you would think that missing out on 200 potency from a lost GCD is negligible, but it does add up over an encounter. Also, gap closers generally aren't available while bound, irrespective of job. Only effects like TW and IR negate that effect.