Fell Cleave's value isn't what it says on the tin because it's a GCD. The net potency gain has to factor in the GCD action that it replaces.

When FC replaces HS:
50 gauge = 1.2(500) - 1.2(150) = 420 (84 potency/10 gauge)

When FC replaces SS*:
50 gauge = 1.2(500) - 1.2(190) - 10 gauge
60 gauge = 372
50 gauge = 310 (62 potency/10 gauge)

When FC replaces SE*:
50 gauge = 1.2(500) - 1.2(270) - 10 gauge
60 gauge = 276
50 gauge = 230 (46 potency/10 gauge)

When FC replaces SP*:
50 gauge = 1.2(500) - 1.2(270) - 20 gauge
70 gauge = 276
50 gauge = 197 (39 potency/10 gauge)

On average, you have:
50 gauge = [420 + 310 + (230)/3 + 2(197)/3]/3 = 313 (63 potency/10 gauge)

(*As a caveat here, under the effect of IR, gauge becomes more valuable, so the efficiency of these moves change because they generate gauge. For example, when you replace SS with FC in a IR window:
25 gauge = 1.2(500) - 1.2(190) - 10 gauge
35 gauge = 372
25 gauge = 266 (106 potency/10 gauge)

Ordinarily, you would expect the efficiency to double under IR, as it does when FC replaces HS (168 potency/10 gauge), Onslaught (120 potency/10 gauge), and Upheaval (360 potency/10 gauge). This is not true when FC replaces a gauge generating action, so Onslaught actually becomes more potency efficient than FC when you clip one of these combo steps out of the window.)

Onslaught:
20 gauge = 1.2(100)
10 gauge = 60

Upheaval:
20 gauge = 1.2(300)
10 gauge = 180

As you can see, Fell Cleave is in fact only marginally better than Onslaught in terms of potency efficiency. In fact, under the effect of IR, it flat out beats it when performing an additional FC clips any combo action other than HS in the window. FC's real strength is in fitting in as much potency in a window as possible.

The take-home message, though, is that you should never feel bad about using Onslaught as utility. You are inevitably going to lose more dps from a lost GCD than you will from being marginally less potency efficient than FC (3 potency on average).