Before people start saying stuff about oathbreaking, let me remind all that Greybeards are supposed to use their voice in times of true need:
Contrary to popular misconceptions, the Greybeards are not, strictly speaking, pacifists; the Way of the Voice teaches only that violence is the least of the Voice's uses, and it should be used for that purpose only in times of "True Need".http://www.uesp.net/wiki/Lore:Greybeards#cite_note-Hrothgar-6
He used it originally to fight in the great war, then again to free Markarth, then the controversial one with Torygg. That to me is true need, because he used it to rally men to his cause, which in my opinion is for true need. To make Skyrim independent and free of the WGC. You can argue against it, but there's no official rule for what true need is, so its up for interpretation. More Nords would join his cause if the leader of a rebellion used the power of the heroes of old to accomplish a goal to be like the old honored Nords. There lies the true need, more men for what he considers a good cause. Arngeir obviously forgot about the true need part:
http://cs.uesp.net/index.php?game=sr&formid=0x0003f884
Ulfric to me seems to be the one who is following the original purpose of the Greybeards. He still seems to follow it now, as he says he knows its not to be used lightly:
http://cs.uesp.net/index.php?game=sr&formid=0x000bfc41
http://cs.uesp.net/index.php?game=sr&formid=0x000bfc43
http://cs.uesp.net/index.php?game=sr&formid=0x000bfc46
He may not realize it, but really, using the voice for troubles like this is what the greybeards were supposed to do.