Well, if you considered the history of the game, a large number of primaries had burns. All of VoG's primaries, all of Crota's, even PoE's original Primaries had burn variants. Then you have Exotics like the Vex and Necrocasm, and now the super cell. Sure, not a super large list and yeah, most aren't relevant now, but the point stands. Most primaries are kinetic, but why are ALL special and heavies past blue elemental? Why not kinetics? I'd use them even in Burn modifier missions, it might make them feel difficult and not painfully easy you know?