The issue is this. You log in to the game and the lobby server has to obtain information from both the JP worlds and the NA/EU worlds. This is undesirable behaviour. During periods of congestion (after patches, after server downtime etc.) you will have over 200,000 people attempting to log in.

If on the launcher you were able to select which world realm you would like to connect to, when you launch the client the client would only poll that regions server, leaving the other realms lobby server alone. Instead of 200,000 players polling the JP world list you would have only 100,000, net result, the JP lobby server isn't so overloaded.

I am on Durandal, I have zero interest in polling the data centre in Canada to find out if their worlds are up and accepting connections, so a simple update will assist later on down the line and enable a smoother experience for your customers.

/edit - title is wrong, I meant to state separate regions on launcher.