I, for one, don't like the idea.

Given the number of Link Shells across the servers, there will be many instances of LS's having the same Logo/shield.
Now what are you going to do when Link Shells X,Y, and Z with the same logo are merging from their respective servers into one super-server.
Which one gets to keep the old Logo? Who gets a new one?
Tough luck for those that might have to change their LS websites.
Price you pay for a populated Game World and Progress, right?

We can also talk in-game economics,
Currently as more and more players approach lvl cap the in-game economy is beginning to become inflated.
With a higher population of End Game players due to a merge, would fuel inflation further.
Yes, it can be argued that more higher level crafters equals more competition, and lower prices.. But that's only on NQ Items.
As there's More End Game players competing for a marginally increased supply of HQ items, and with no Money Sink in-game. (Fast Travel, Gambling on Chocobo Races/Fights in the Gladiator Pit, Housing)
They're all Gil'd up with nothing to spend it on, so they're looking for better equips, and just so happen will progressively pay more and more for the items.
Let's not forget the market disparity between prices of the same item across servers.
After a server merge, there will be some who gain the benefit of moving from an over-inflated game economy to a more controlled one.
Though there'll be similar numbers who'll find the opposite is true.

While I can appreciate peoples arguments FOR server merges.
I have to say I'm probably one of the few on a Server dominated by Japanese players (With English clearly being in the minority) that would be against such an idea.
Even so, I've managed to find a great LS with some fantastic people to play with, and even a few RP'ers.

Now as for a Real World Economic point of view.
SE already own the servers. Kinda pointless shutting a few of them down and having to port all the character data across to different servers.
What becomes of the empty server space? Rent it out to who? Risk sites not controlled by SE being hosted and potential sources of security breaches.

*OOP's what was that? You wanted to choose which new Server you go to.. What if you're not given a choice?
Some of your friends are on a different server due to you being a Euro player and they're NA?
Sorry but that's how it was decided to distribute people from the now closed servers across the remaining worlds

*Potential situation arising from Merge.

SE are expecting an influx of new players to come with the, eventual, release of the PS3 version.
Perhaps then you'll see the Cities and Camps alive with players.