Ive noticed as Blm that if I use an AOE fire spell, I generate more enmity to whichever mob I targeted. Even though the spell might do equal damage to all mobs hit, it doesnt seem to do equal enmity to all mobs hit. Therefore if the tank marks one, I can target my AOE spells against the marked one.

Marking multiple targets before the pull can be done without even stopping your movement to the pull, more less if you just mark the one you are targeting.

Bottom Line: There are several reason to mark. The only reason not to is because the tank is lazy.