[censored] !! Hammerfell fought them off alone when THE EMPIRE abandoned them, tried to [censored] them out to save their own skins. Disgusting.. And they'd do the same to Skyrim in a heartbeat. The Empire serves nobody other than the Imperial elite of Cyrodiil.
That was 30 years ago and the Dominion has had plenty of time to gather its strength, whereas the force that invaded Hammerfell had been but a fraction of what remained of their forces after the Great War. I doubt anyone in the Empire was happy with the series of events that led to such a moment, and indeed is proven by the Imperial volunteers who fought on behalf of Hammerfell even as the Empire officially signed the peace treaty with the assumption the provinces would do the same. However, for you to say that the Empire severs no one but the elite of Cyrodiil is dishonest and ignores that the Imperials sacrificed their homeland to say "no" to the Dominions demands for a much larger chunk of Hammerfell than was signed away during the treaty. Lets' not forget that the entire war took place in the Imperial home province and that none of the other provinces in the Empire had known the war to such an extent or fought as hard. No one likes a lost war, but for you to assault the Imperials in such an unfair fashion is dishonoring everyone, Imperial and Nord alike who died fighting the Dominion.
Two strong allied kingdoms (Skyrim and Hammerfell) would be far more of a threat to the AD than provinces ruled by a weak corrupt Empire, Nords fight better when they're fighting for a real cause, the Empire is not one anymore..
The Empire consisted of Highrock, Hammerfell, Skyrim, Morrowind, and the Imperial province and the Dominion still defeated it, leaving it bloodied and battered. Under what logic do you think a fraction of that same strength would be able to defeat the same host that forced such a sever peace treaty upon a much larger one? Further, why would Hammerfell ally itself with Skyrim? Hammerfell called for assistance against the Dominion, and no one in Skyrim answered, including Ulfric himself. Mankind’s only hope is to present a unified line of defense against the Aldmri Dominion, we stand no chance of resisting them if our strength is divided, and that is exactly what they are trying to do. Would you knowingly fall right into their trap?
You claim the Empire is weak, but how much weaker would Skyrim be if it were to stand alone? There is corruption within the Empire, true, but no more so than any other government. Would you trade one corruption for another simply because it is based closer to home? Yes, there is corruption, but there are also honorable men as well, which also is true of any government.