Thing is it adds utility, for example:
http://wiki.ffxiclopedia.org/wiki/Sleep_Arrow
http://wiki.ffxiclopedia.org/wiki/Demon_Arrow
http://wiki.ffxiclopedia.org/wiki/Paralysis_Arrow
http://wiki.ffxiclopedia.org/wiki/Acid_Bolt
http://wiki.ffxiclopedia.org/wiki/Bloody_Bolt

These constantly applied the status effect on the mob, for example the RNG would shoot an acid bolt and it would weaken defense, they keep shooting it (though its a waste to) it'll always "weaken defense" basically restarting the def down timer, whereas using a potion isn't as efficient as doing damage + status effect.