First of all you will never see a mass movement of 150 players.
Why not? They just saw half of their buddies disappear, including the goon that was about to drop troops, and the sky in front of them is now devoid of enemy aircraft. Why wouldn't they want to get back to that fight?
If you now have 8 targets in front of you and then you have 4 targets, you're not going to miss the other 4 on the other side. You will still be in the same spot on the map, the same fields will be on fire, your objective is still the same. You'd gain absolutely nothing by switching sides at that point. There wouldn't 'be' a 'better fight' on the 'other side' since it's the same exact status of the war with the almost same exact numbers of players on both sides.
Ok, I think I see where the disconnect is.
You're assuming an even distribution of people across the map on the server. Have you not noticed this is never, ever the case? You had 8 targets in front of you. They were all squaddies. They all switched at once. You now have zero (0) targets in front of you, because the server split. This isn't annoying?
The enemy horde you're moving in on was comprised of members of 3 squads and a half dozen single people. Now suddenly instead of 25 planes for you and your 15 friends to play with, you now have 6. More importantly, THEY now only have 6 on their side. This isn't annoying?
Even assuming your perfect ideal, where through some miracle 50% of the people from all sides in the local area disappear, the situation was you upped to defend an airbase with a dozen other guys against 30 hordelings. It goes from 30-12 to 15-6. 12 guys can fight their way out of a 30-12 a heck of a lot easier than 6 guys can fight their way out of 15 planes.
TNG of thinking perhaps? 
Heh, funny. I loved TNG. I wouldn't call it groundbreaking or great, compared to TOS. It was more of a rehash of the formula that while decent, brought very little new to the table, and many who liked the old show found it vastly inferior.

And to everyone horrified with the idea of teleporting: This is only going to happen after 200 players on average will join the server. That's going to take a looong time to happen. Furthermore if so happens that players like this so much that playerbase grows to 600, a third server can transparently be created using the same method. Then fourth, then fifth.
Again, it reaches 200. It splits into 2x100. 75 of those guys log out of the second server and back into the first within the next 5 minutes. It doesn't matter why they did it, they did. What happens when 26 more guys come into the first arena?
Wiley.