I usually only switch country when I see an enemy country is in a position where it affords a super rare opportunity to attack a field or see things you don't see often in the arena.
A few times, I have switced country when I answer the call for troops or leveling of a field/town and find the people who called for help won't advance on the field.
I have also switched sides to fly the wing of a whiner and not give check six calls when he's in trouble and then type "lol" when he get's shot down.
Any country is the same as the next and it's good to meet, befriend, or sometimes even anger all who are in the game because you could likely play for years and not meet a significant percentage of the players if you fly the same side and sortie type every time.