What.
The Imperial City is in Cyrodiil. The Dawnguard DLC doesn't have anything to do with it.
Is this about how there "just happens" to be a Moth Priest in Skyrim? It's not that much of a stretch, Moth Priests are always searching Tamriel for Elder Scrolls. The only ones that remain in the Imperial City entirely are the ones that have become the ones that have become incapable (whether by blindness, old age, or insanity) of searching for more.
Also, and I know this is off-topic, but why do people feel the need to self-kudos? No one is keeping score, you know.
You mean it like this?
You would visit the Imperial City for said Moth Priest (assuming he wasn't in Skyrim) and then the quest would proceed normally.
Pros:
You would visit the Imperial City.
Be able to see and hear all new rumors and facts (Kvatch and the other cities, the Arena, Martin Statue, White-Gold Tower, etc).
More clues about the current situation of the Empire and the Aldmeri Dominion.
Cons:
Thalmor everywhere and a very small number of quests (if any at all) which would be a major let down.
Too much trouble then it is worth for Beth, considering it would be just a featured location in the Dawnguard DLC.
Watchu talkin about, Willis?
This is off topic, but there is mention of the Imperial City in the game such as the 1st Edition Book store. It would be cool to have that has a place to visit. The world is big enough through and there is no need for the Imperial city as you already have the College in Winterhold and the College in Solitude which I think is consider as the capital city of skyrim.