Sunsingers can generate a LOT of orbs since all their weapons generate orbs when used with radience active so I think you're wrong. Also being able to kill enemies over a period of time and still generate orbs is very useful in the VoG since a lot of the time enemies are not clustered tightly together. Fireborn is just an additional perk of being a sunsinger. If a hunter were to die during the Atheon fight in hard mode you would lose 1/6 of your fire power but a sunsinger would be able to continue damaging Atheon OR take out the surrounding supplicants and be able to spawn more than enough orbs to fill a super meter, something no striker or Voidwalker could do since their supers rely on groups of enemies and since supplicants tend to be scattered around in the Atheon fight, sunsingers are super useful. In my original post I did say that sunsingers were less useful in the fights then other classes but after a bit more development in our organisation, my fireteam can have almost constant orbs of light if we use our supers are used at the right times. In the Crota's End raid, self reviving is less useful but I'll be dammed if radiance isn't useful during the Crota fight for clearing out ads and it is still very useful to quickly self-rev is taken down during the fight because having all six players gunning the oversoul is way more useful than someone who can use their situational super attack.