I am playing on very hard. My Small Guns is at 60%. Some out there seem to have a pretty high DT and/ or damage reduction. Even headshots sometimes seem to do pitiful damage. Are there scenarios, when you prefer disabling limbs first, instead of going straight for the kill, like with headshots?

When is crippling limbs useful?

I found it very useful against Super Mutants in Vault 87, during my first play through ever. The only weapon I could really use against them, was the Stabhappy. A combat knife that hurts limbs more, so I was easily able to knock the weapon out of their hands, leaving them no real way to hurt me. I wouldn't have made it to the GECK without it.

Anytime the mob can get to me before I can kill it. Deathclaws anyone? There is a crafted weapon which is almost guaranteed to cripple limbs. Both the Dart Gun and the Railway Rifle can do this. Although the Dart Gun is specifically for crippling legs. Put a shot in each leg and the deathclaw can be whittled down at your leisure.

http://fallout.wikia.com/wiki/Dart_gun

http://fallout.wikia.com/wiki/Railway_rifle