The range on hawkmoon is ridiculously good with hammer forged, but regardless... there are a large number of outstanding exotic primaries, but very few of them justify the slot to any extent in PvE over what you could use the special or heavy slot for. I would identify the Vex Mythoclast as the lone example.
I use Hawkmoon in PvE an obnoxiously high percentage of the time, and even I can admit I'm playing less efficiently than if I were to run one of the exotic fusions, IB, Gjallarhorn, Thunderlord.
The waters are considerably muddier if you have Fatebringer or a great rolled 331 Hand Cannon. And of course, during a Nightfall you want a raid weapon with the right element in your primary slot which means using your exotic slot for something else. Except again, of coure, the Vex.