Poison/Dark is the only defender that has really clicked for me. It helps A LOT that there are only three powers to cast on teammates, and one is a resurrect, and neither your spot buff nor your spot anti-mezz are really anything to write home about. Everything else is aimed at the enemy. The debuffs do not require fiddling with 'anchors', you just aim them at something and fire.
I have many level 50 characters on TB. There are other characters that solo well, or that run story arcs well. That isn't what this one is for. But if you are up against an annoying archvillain, whether it's Mary Macomber, Reichsman, or Nosferatu, I will bring this character who exists to help other characters trivialize tough encounters.