"Just register on the NA server if your friends are NA" is not an acceptable solution, for a variety for reasons.
The first is the one DV outlined: It's standard for off-timezone players to be weekend warriors, playing team matches on US servers when they have those hours available, and practicing in their local prime-time for the rest of the week. If you register on a different server for the purpose of playing with friends on a different timezone, you're doomed to life in a ghost-town because nobody will be playing there when you try to log on without your team.
The second are the non-team relationships you lose. What if I decide to bite the bullet and join my old team on NA, but other friends count the cost too high and stay in SEA; those relationships are now gone. What if I'm dedicated enough to the game and my team to do that, but then want to bring an RL friend in? Do I make them join NA too?
The third is something very evident in Starcraft 2, and that's region dominance. There's an effect where one region gets a reputation as having the highest level of play, and begins to cannibalise the others. Good players on other servers migrate to the most prestigious one to find better enemies and friends, and this has a knock-on effect that is very quickly felt by even casual players. SEA in starcraft, for example, is a ghost town. The majority of Australian players above a certain level play on NA or KR/TW, and that means that local matchfinding is painfully slow and ranks are meaningless. Even the highest tiers on SEA are "king of the desert" awards compared to bigger servers, so more and more players jump ship. Region-locking actually kills the very scene it's supposed to protect.
I understand that all of these can be circumvented by the 'ol "it's free to play! just make an account on both servers," but that is in fact the greatest betrayal of trust of all, because we are then asked to pay twice for the privilege of accessing the same content on both sides of a divide that should not exist in the first place. If a player opts not to pay twice, joining an off-server becomes an enormous chore, because they are forced to operate with inferior content for what they see to be no good reason. That experience fosters nothing but bitterness in the playerbase.
Gouging the people who are most invested and excited about a game is not a smart idea if you want to build trust and keep that community around.
Edited by Belisarius†, 18 May 2012 - 03:12 AM.