He is Skyrim's last hope, in a nationalistic sense. With the Empire, Skyrim may very well survive, but it won't be "skyrim" if the Nords have turned into weak, submissive servants.
They're far from submissive servants under the Empire. The Talos Priests all speak about how they will continue to defy the Thalmor in their hearts. With war just around the corner, are the Nords going to sit there sulking about things that no longer matter once the White-Gold Concordat is torn up and Tamriel decends into war?
Ulfric is far from Skyrim's last hope. You follow a politician who claims that he, and he alone is the last hope? You speak of nationalism, yet not one true High King of Skyrim would ever put themselves above their men. It would have been
"We are Skyrim's last hope", you Stormcloaks fight with mismatched equipment, sometimes poor grade of weapons while the Jarl of Windhelm gets a fancy ancient royal looking blade made? Not even the Imperial Emperor Titus Mede used better weapons than his men.
The only weak submissive servants are the Stormcloaks, so wrapped up by Ulfric's words they don't even think about the lies he fills them with. According to the Stormcloak Officers, High King Torygg and the Jarls who supported him were to blame for the treaty in Skyrim (Which is odd since no one was against Torygg until Ulfric challenged him) and the Emperor surrendered when the Imperial City was threatened.
Even Galmar grows tired of Ulfric near the end of the war, so obsessed with his image.
http://cs.uesp.net/index.php?game=sr&formid=0x000d1e0e