Exploring enemy faction zones to adds to the lore and realism of the game, if the Aldmeri Dominion and the Ebonhart Pact are at war why limit their conflict to one measly province? That's not war. Sure, taking the Imperial City is their primary objective. But in WWII the Allies didn't spend the entire war trying to get to Berlin, there was battles everywhere.
Let us sleuth around enemy territories. If players are feeling bold enough, let them send raiding parties. WoW did it years and years ago, so can Bethesda.
Cyrodiil is more than just a "measly" province. According to the maps shown at E3 its land mass is equal to if not greater than then each factions area. Its in the center of the game world.
http://4.bp.blogspot.com/-C4NjhKC4DTc/T2WnJdxPMfI/AAAAAAAAE3E/fUETZV9g0A8/s1600/ElderScrollsMMO2.jpg
http://www.geekworldradio.com/pics/imagegallery/images/Bethesda_Booth_TheElderScrollsOnline_MapOfTamriel.jpg
Besides I don't think its been 100% confirmed that you can't go to other faction lands. There won't be any quests or anything for you to do there but I'd be surprised if there's absolutely no way to get to an enemy factions land.