Repairs is the primary reason for it. Unfortunately you take damage to your gear just by being in combat and using any form of action at all.. Doesn't matter if you're hit, or even if you lose HP, your gear degrades.. There's no point in wasting gil if they aren't necessary to kill. Likewise the trash mobs drop nothing, no items, no gear, no seals, no tomestones, and not even experience because you're capped at that point. With nothing to gain from killing them and money to lose (not to mention time essentially wasted) there's just no point in it..

Mind you, this is coming from someone who is fine with taking it slow. I don't mind doing runs normally, and I don't bitch at people to do speed runs. I just see the reasoning behind it, and if my group wants to speed run I'm okay with it, because I understand exactly why they want to do it.