Sunsinger is really only good for Nightfalls and nothing else. You also don't see many warlocks running sunsinger and PvP because it doesn't offer much other than an armour boost. Nova bomb is so overshadowed by everything else that almost nobody runs it. Again stormcaller is just as good for PvE as a Nightstalker, but Nightstalker can have more uses. Nightstalker with blood bound is pretty much the equivalent a stormcaller except it can get the job done much quicker as well as tether a boss to do more damage. There are some places stormcallers shine, usually when there is a larger group of mobs that nobody can tether them and get them all. Stormcaller; however, just can't keep up with Sunbreakers. The damage they deal so high as well as their armour that is is to hard to face one as any other class. Sunbreakers will just destroy anything that can get close to them. Stormcallers can be good, but they can easily be taken down by some well placed fan fire or another super. Again, this is all based on what I have seen. If you are successful with a warlock, use it. If you find ways to make a warlocks stand out, use it.