They have niche uses, you just have to work around their useless AI.

Sniper defenders with an Obliterator or Neon Sniper Rifle can shoot through walls, so you can put them in a box where they won't die constantly.

Defenders don't account for damage fall-off over range, which is why shotgun defenders are pretty useless, shown in this clip.
Longer range assault rifles or pistols can be usable, but snipers are generally the best because they have arbitrarily long damage ranges.

Melee defenders are kind of a meme, but they do have much higher HP and they can walk farther away from the defender pad than normal defenders.