Wardens are needed during Blights -and between them as well. As stated in the Warden's creed: In Peace, Vigilance. They keep tabs on any darkspawn activity that might be a pre-cursor to another Blight. Allowing the Wardens to slack off between Blights will surely negatively impact their combat readiness and combat effectiveness -if they only spring into action when a Blight is imminent.
But Duncan's attitude "the end justifies the means" is a dangerous one; and one that Loghain apparently abides by as well.
At Ostagar, Alistair even admits that Duncan once said that the Wardens do whatever it takes, to end a Blight; and that could mean some very heavy measures. Yet Alistair seems to personally shy away from doing whatever it takes to get the job done; he wishes to commit the very limited resources the Wardens have available to them, to fight and win every single battle against the Darkspawn threat. And yet, that could inevitable cost them the ultimate victory. Equally, if Loghain wins at the Landsmeet, Anora's father will have won that battle, but he will lose the war.
I concur that Alistair's idolisation of Duncan is a major pain -especially when I play the Dalish Elf Origin. All I want -having failed bringing back Tamlen- is for my clan to be merciful and kill me when the time comes. But the Keeper throws me out, because Duncan demands it. (I wish I had the option to renounce my Dalish roots). From then on I try my best to let the humans suffer -and Alistair is nothing but a major pain in that story. So, my Dalish Elf generally dislikes Duncan.
Duncan's murder of Ser Jory is another thing that cranks up my dislike of Fereldan's GW leader. So Jory gets cold feet -and he can be called stupid for not grasping there is no going back; he believes he can return to his wife after the Blights'been defeated- that is no reason to kill him? Selfdefence? Pretty weak. Jory's probably frightened out of his mind, they can probably not talk him down in laying down his weapon, so his death seems inevitable. But give Jory some credit: he fights Darkspawn up close and personal -can't believe that's a lovely teaparty. Then he's told he's going to have to drink their blood -and everyone knows that close to suicide. He then sees Daveth croak in a most alarming way; yeah, I'd get cold feet too.
Just like Loghain's is so blinded by his hatred of Orlais, Duncan is blinded by the sole purpose of defeating the Blight. Anything that stands between them and their goal.....
In all, I generally respect Duncan for his dedication, but it's mostly Alistair's selective blindness to Duncan's lesser noble attributes that rattle my cage.