They don't require that much more support than a ninja or a pld would, really. They can wear every almost ever haste piece(boots is the only exception and they have their own) that a ninja can; so with the same songs you'd be giving a NIN, they can keep up their shadows with the same amount of ease a ninja could. The timers would be the same or at the very most only slightly worse(1 second, maybe 2 seconds at the most), so blink tanking really isn't that much different than with a NIN. They get massive amounts of enmity gear from their relic/AF, so they can compensate in other places because they get a large chunk of enmity on their armor. Unless your mages really suck, you'll be getting haste/march, along with slow and elegy on the mob; which is usually more than enough to maintain shadows.
Plus there's the extra benefit of being able to wear most of the armor a PLD can, when you do get hit without shadows, you can actually take the blow much better than a ninja can. You also have the benefit of the tank being a capable damage dealer, just like a ninja can, but with more versatile placement on the skillchain chart in case you need them to participate in a skillchain. The only real advantages that NIN's have over WAR's is that Utsusemi: NI gives them 4 shadows instead of just 3 and NIN's can get a little more -% physical damage due to the arhat's set; but that really wouldn't be enough to keep a competent warrior from being able to tank effectively.
In the really big fights you'd have a couple tanks anyways, so saying that it'd be a drawback for them to need a supporting tank is really untrue, when even the "real tanks" would have backups in those fights.