We see the events first hand. During the civil war, no reinforcements are sent. Infact when Ulfric wins, he isn't overtly concerned about reprisal outside of the camps in the hills.
Is it possible that the Empire will send reinforcements in a DLC? Yeah, but IMO that would detract from the story and drag you back to a part you've already done. I'd rather move onto gaining Hammerfell or fighting the Thalmor, personally.
Correct, he is not concerned about the camps, but he seems concerned of the possibility that the Empire want to take Skyrim back. At the end of the day the Stormcloaks are a decent army, but they are much less powerful than the Empire even in its current state, both in terms of resources and manpower.
Is a Hammerfell/Skyrim alliance beneficial? Yes, I think it would be. Is it better than an Empir with Skyrim? We do not know.