Prolonged usage of Sword/Board is being phased out for more aggressive and offensive Greatsword play post 4.0 as it's becoming more and more apparent that a component of DPS needs to be involved to effectively maintain aggro on top of regular taunts. 

Is that to say that Emp Prov Tree is dead and buried? Not necessarily, I expect it to make a come back in 4.5 content where Eternal Bastion seems to require a larger degree of survivability over DPS output for selected Templars in the alliance. 

Could you effectively deal damage (Meeting instance requirements of minimum 1k crit, 3k accuracy) on sword board though? If your gear allowed you to be at 1k crit and 3k accuracy on sword and board I think you should reconsider what role your weapons play, as in the grand scheme of things your greatsword will still do theoretically superior figures in terms of DPS. 

Furthermore, if you were going to be going prolonged and sustained usage of sword/board to tank said instance, assuming you're doing it in Emp Prov tree you should be in a HP Set which essentially rubs out you being able to reach the requirements of the instance. 

If you swap to shield and board, the idea is that you're attempting to reduce damage intake in some way shape or form, not to potentially be doing more damage than with greatsword up. Exceptions would be running out of moves to weave on greatsword and needing to swap over for Shield Bash etc. 

Similarly, by swapping to greatsword you're doing so with the intent of dealing large damage figures, which is also reiterating the idea why your greatsword should always be slotted with Crit Strike or Crit/___ Composites. 

Taunts won't get you across the line alone, classes like Assassins whom have a much more linear DPS in that it remains steady and doesn't fluctuate as much as you taunt CD's do. Note how long most CDs to bigger Taunts are, then note that their DPS just linearly goes up, eventually something is going to give way and it's going to be you sitting there having lost aggro with taunts on CD and them tanking.  

"But what If I swap to greatsword, gear macro set to a mainstream DPS set and burst between taunt cycles?"  ಠ_ಠ, you could, but that would be trying way too hard to achieve essentially what you could be doing in DPS spec but better.