Lightning will be getting a slight defense buff in 5.6 via 5% DR when you have the dionized thing on you, so basically anytime you have your or another sorc's bubble on you.
But ya, as far as class balance for defense/survivabiltiy in PvP goes it's pretty crap, Juggs, PTs and Sorcs are bottom of the barrel, though we Sorcs can barrier so unless you get staggered nets it's possible to survive longer under focus fire, where as Jugg and PT Dcds can easily be bursted through, but after you use barrier you're pretty much dead, you can phase walk and heal up if you placed it correctly but you're still likely to be dead if they want you to be. Even with the nerfs to dcds Mercs will get in 5.6 they still will (probably) be far tankier than other classes, with Snipers being the closest.
With that in mind if I were in charge of class balance for dcds I would either gut all dcds for DPS specs so all of them quickly die under focus fire (which might not be a bad thing) or buff/change dcds so all of them survived longer and there weren't any classes or specs which easily outshined the others.