The Empire is filled with diplomats and politicians, not Captains and Warriors. If they settled with the WGC, then what makes you think they would just join Ulfric. At the time of the WGC, Ulfric was with the Empire.
If Ulfric is such a great warrior, and his Thu'um so mighty, the Empire wouldn't ever have thought they couldn't take the Thalmor and agreed to the WGC. Ulfric tried to fight the Thalmor years ago and failed! He was captured, tortured, and gave information to the Thalmor. Now he acts as if he and he alone can stop them? Somehow I doubt he has the skills to back up his claims.