Personally, I use defend most often in random encounters, because they often start with my fighters too far away to attack anyone. I rarely use bows on my fighters, and even more rarely on my second slot fighter, who I generally make a slith and give tool use to - that means I have fewer skill points to spend on missiles, while my first slot fighter (often a nephil) is decent with them naturally and generally has more spare skill points to spend. Either way, most of the time, I use defend because melee and pole weapons do more damage than missiles, and the defend option lets the enemies come to me so I can hack them to pieces.