I have been in a few combines.

Some are dead. There may be lots of players in it but there is no interaction going on.

Nothing in chat, no knowledge how to do SnD, no defending each other, etc.

There are so many CEO's in this game who dont know anything about how to run a combine.

They collect just a bunch of individual players and that's it.

Hardly any progress, players get bored with the game soon and go MIA.

If you are an active player, go look for a better combine.

If you have a smooth running combine where teamwork is normal, it will jump up in the rankings.

And you will have a lot more fun out of the game.

well, from a bit of time ago, i had the intention of writing two lines of considerations on this topic. Now i have found a bit of time to write.
These my guesses are all related to the international version of Soldiers Inc, running on site "plarium,com".
(so, may be not applicable to fecebook running and russian running, of this social online game)

Loyalty to one combine make sense only if the combine in itself is one of the top combine in the game. and the player has an advanced gaming experience. In this case, the player draws huge and significant benefits from loyalty to combine which it joined. 

Otherwise, if the player not have a significant gaming experience, it is definitely a good choice to pass from a combine to another trying to get as much as possible from every experience.

Because, rightly, each player must think of himself (ie to save his own skin), the concept of loyalty is extremely partial. In most cases, it remains in a combine until he has his own personal gain.

Last but not least, as wittily previously written by Flea Ridden, a huge problem, really serious, is that " There are so many CEO's in this game who dont know anything about how to run a combine ". So, in real many case, players with less experienced, do not have any help from their own ceo.