If you could send soldiers like you could anything else, it all balances out. Think about the possible scenarios....
1. a fam member is starting to get raided. hes not online. Quick send him soldiers whoever is online!! One of the sucky things about this game is what happens when your not active. You can wake up and find you got raided into pmode. This would actually help the people who just cant be as active as others, thus increasing their enjoyment of the game.
2. So you sent your soldiers to Empire A but now you have none. Your now a vulnerable target until either a) you build more soldiers or b) someone else aids you some. Aiding soldiers can be used against you as well. An attacker can try to bait the active players into sending away soldiers to the one empire not online as a ruse into raiding them instead.
3. The soldiers you sent Empire A got slaughtered along with his own force. Hes still getting raided. Who has extra soldiers around here? Oh yay so and so the resourcer just logged in.
Which brings me to point 4.... You now have a new type of resourcer, the soldier farmer. You could have an empire with a good construction make soldiers for everyone else with a lower con. Soldiers could get passed around with the family bank. Making for some interesting strategys.