Is there really an outstanding exotic primary?
I mean, I do love my hawkmoon, but it also has its flaws (as a scout rifle lover, range is one of them, for example). Mida is really cool, but not THAT MUCH better than a good legendary scout. Bad Juju in an obsidian build is funny, but pretty meh otherwise.
I'm struggling to find what primary exotic I would really "need" to use in PVE...
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.
edit: I'll include two caveats
1) situational ideals, like Red Death on Crota
2) build ideals, like Bad Juju with an intellect/Inverse Shadow/Gunfighter/Obsidian Mind kind of thing