Every third thread on this forum is dedicated to whining in some incomprehensible way about why they hate that the faction borders are closed. I'm getting sick of it, so here's a thread designed to praise the merits of that decision, and thank the developers for sticking to it. Here goes:
1. It creates realism.
The factions are supposed to be at war with one another. In a real war, border locking takes place almost instantly. Citizens of one faction don't just wander across the borders into the other faction territory and look for shops or quests. That's not war.
2. It places emphasis on the goal.
The factions in this game are supposed to be fighting over the Imperial City, trying to take advantage of the power vacuum there. It sure would be pretty terrible if there were very few players actually in Cyrodiil fighting over the City. With the faction locking, pvp'ers will focus their attention on the goal, and that makes the game far more interesting.
3. It balances the interest.
Certain people really just love to rift into enemy territory and gank lowbies while they try to level and quest. I have no idea what the appeal of that is, but a lot of people seem to enjoy doing it. Those people are often quite vocal. However, a huge mass of people prefer to be left the heck alone while they are questing and levelling at home, and don't like to see gank squads running around picking people off. So, protecting the home zones answers the call of the second group, which for some reason has often been the group left out of the discussion. The pvpers and gankers still have an open province to play in if they want to, you can open pvp in the entire province of Cyrodiil. Thus, each group gets a little of what they want, and balance is important to maintain a large population in the game.
So there.
Faction locking and RvR pvp in a warzone are wonderful things. Thank you ZM!
(P.S.
Yes I know you can cross the borders once you hit 50. That seems to be there just for people that want to explore and look around, which is fine. You can't pvp there, and there's probably not much of anything that you can do.)