Wiping due to an undergeared or inept healer, or spending five minutes trying to kill two packs of trash because of piss-poor AOE, isn't much of a timesaver. Really, NIN+MNK is one of the DPS combos that makes my heart sink a little as it's about the worst AOE you can have, and it's probably just as fast to pull one pack at a time and single-target nuke the mobs than have them dinking away with their AOE that hits like handfuls of wet noodles.

Really, it's only when there's a well-geared WHM (dat holy), or at least one BLM or BRD that I feel it's that much of a time-saver to pull multiple packs. Besides, if you're a DPS you probably just queued half an hour plus to get in, five minutes extra on the run itself should be nothing by comparison - if anything, it's the tank with his instant queues who has the most incentive to go fast, as it proportionally shaves a lot more time off of the queue plus run time.