In Stronghold and Crusader, archers (and Arabian bowmen) get an increase in damage when they target units with heavy armor (pikemen, swordsmen). Against light and leather armored troops they gain no bonus. Since braziers are free to place and they have a large aura to give bonus (5-6 tiles approx.), they are worth placing. Besides, fiery arrows have fun trails :)
In Stronghold 2, braziers have much smaller aura and cost 50 gold to place. However, they increase damage considerably, against light troops too. These arrows have much smoky trails.
Since units are common with heavy armour and pitch ditches are a good last resort of defense (to a lesser extent in SH2), I recommend placing them. In Crusader you can just spam braziers and make a good defense of archers. Note that crossbowmen and tower ballistae won't benefit from them.