Onslaught is a resource based gap closer, like SAM's Gyoten. It has a cost to offset the negligible recast. The point is to increase uptime on demand, not potency. Part of the reason why you see no SAM complaints on this issue is because most melee dps would understand the value of sacrificing a small amount of potency in the short term to avoid missing a GCD ability.

If you reduced the cost by even 10, it would become more resource efficient than Fell Cleave. Even as it is, under Berserk + IR, there are certain GCDs under which Onslaught is more resource efficient (basically, if FC replaces any GCD other than HS in the window). The balance is fairly tight.