Depends how much damage you can do in the 2 seconds it takes to reach your opponent. If you lose 1000 damage from 1 GCD and only do an extra 200 damage from auto-attacks, you'll lose DPS. Not to mention you may have to move more at melee range, which is a DPS loss for someone like a BLM or anyone using cast-time spells.
I wouldn't think too much of it. The only time you should care about auto attack is if you're in Titan HM and feel like building LB on his first phase. The damage loss from moving will most likely never be made up by auto attack damage, unless you lose no damage from moving, in which case do it if you really want.
At the end of the day, however, 200 damage in a 1 minute time frame is negligible at best. You won't notice the damage, nor will it help any situation. The only time it would make a difference is if every single person in the group did the same thing, account for maybe 800 extra damage per minute but even then, still negligible.