you're saying if I was in his shoes and conjured Ohdaviin to fight the battle for me it's still fair? I disagree.
by the same token you could argue that you're fully legitimate in killing a child provided you issued a challenge the child accepted.
Odahviing is not a Shout, I myself disagree that it is categorized as a Shout, you're basically asking somebody to fight for you. Which is not what Ulfric did: he fought by himself. He's simply too strong for Torygg to defeat
an anology would be a nuclear power engaging in warfare against a much weaker nation. Just because you have nuclear weapons in that situation, does it mean you should use them? Not when you can easily win without them. Bad publicity, and hurts relations (the part that really weakened Ulfric's cause from the start).
The sad truth is, that's what happened in World War II: Japan was ready to surrender, but due to some mistranslations (them being Japanese and all), Hiroshima and Nagasaki took the brunt. And what happened? Within 60 years the Japanese export stuff that many Americans import.
Strategically, winning as quickly as possible is the best course of action. It wouldn't take long before Imperial units start to surround him. While you may argue he wasn't fair, he's basically fighting within enemy territory miles away from the nearest safe point.
and, if Ulfric wanted to prove his superiority, and as is later revealed that Torygg knew he stood no chance, I'm fairly certain he could have done so without using the voice. Furthermore, it would vastly improve his standing among the people. What's the most common thing guards say about the fight? "They say Ulfric Stormcloak murdered the High King... with his voice! shouted him apart!" This signifies that it was not so much the death of torygg at ulfric's hands that concerns them, but the use of the voice to kill him.
More specifically, they are talking about how the Shout was said to be so powerful and badass that Ulfric can banish people to Sovngarde merely with his
bad breath Shout. I too, would be impressed if Osama bin Laden can conjure a freaking fire ball from his hands and use it to burn down a president from some country. I won't support him, but I'd be heavily impressed, if I don't call street magic trickery.
Argenir doesn't discuss it with you about Ulfric. He states the beliefs of the greybeards when you mention the blades during the dragonrend quest. Using the voice in combat when one isn't the dragonborn is not something they condone.
I'll see what I can dig up. I've never heard the double-standard way like that. I do remember that he said the use of Shout is kinda necessary in MQ's problem, but he never said anything about only Dragonborn can use it freely