You should be using special arrows to get yourself out of tight spots or to set up combos/opportunities for your party. The only time they should be used for DPS is poison cloud + fire combo.
You should also always be aware of the enemy's bodybuilding and willpower stats and choose your arrows accordingly. For example, don't use a knockdown arrow on something with 80 bodybuilding, use a stun or slowdown arrow instead. Usually only bosses have high bodybuilding AND willpower, which means that most enemies are vulnerable to at least some of your CC arrows.
As a final note, special arrows always have the same chance to apply their status effects (most of them are 100%, some are slightly above 100%). This makes them very powerful early game, but since they don't scale with level or dex, most of them get outpaced by the equivalent mage spells.
