The one problem with keeping all mobs in front of you is when you have large mobs mixed into the group. Since mobs (and chocobos) have collision physics, they tend to bump each other as they try to get closer to you, knocking some of them around to your flank (sometimes even further than that, depending on the size of the group). If you have ranged DPS, it's usually no big deal to dance around a bit and try to get them to line up again, but when you have melee DPS with positionals, you also don't want to ruin their damage potential ^^; So it can be tricky sometimes to determine when the DPS is focusing on AoE damage skills (and thus make it fairly safe for you to spin the mobs a little) and when they are doing positionals (for buffs or because there are other reasons to not do AoE damage skills, like in lower level dungeons where they don't really have those yet).

On the other hand... Those are the times you pop your tougher cooldowns and weather it out ^^;