First, it is a useful perk, and is a life-saver in the Nightfall. I personally prefer Radiant Skin, or Song of Flame if we are all bunched together. But for some missions, where you need someone to take a risk, a Sunsinger with Fireborn is probably the best bet. Also, what if you get sniped in the open when running from cover? It saves a lot of time if you can self-revive and run back to cover. It seems to be very useful when fighting Supplicants in the Vault. A warlock with Sunsinger can easily prevent a wipe. It has it's uses, so don't hate on it too much. With that being said, there are plenty of people who don't know how to use Sunsinger correctly, and they hold onto it for a ridiculous amount of time, so I understand the caution. Before you kick them, ask if they could use Song of Flame or Radiant Skin instead. If they say no, probably give them the boot, because they probably use it as a crutch. If they say sure, or that they'll switch for at least part of the mission, then I say let them in.