Well, if it was Sparta, to start with, there would be no silly teleporters nor energy shields, right ?
Then the whole game is based on your own city as a starting point. This is where your troops are built, and where you hide them when you're offline.
Allowing for troops to go from a city to another would mean their ranks would grow thinner and thinner after each battle until you've lost all of them.
I'm not saying it couldn't still be done, but I like to be able to send my reinforcements along.
Now we have been discussing recently about the opportunity to send troops directly from another city where they could be garrisonned (either defending after you sent reinforcements or liberated it, or besieging, for instance).
Tactically, I agree it could be interesting to send troops to a distant city, then attack a target from there. Of course it's not implemented, but at least it has been discussed and globally considered as an interesting idea.
Now what's the difference with your own idea ? Well, it's in the word "garrisonned".
What you're asking is they fly from city to city like a brood of crickets.
But when you've raiding a city, your troops are always automatically returning you the goods they have plundered, that's why your next raid can only start from your city.
This is because your troops never stay garrisonned there when you're raiding. Obviously, historically speaking, I can't see how your victims would acclaim their bullies to stay defending them...
That's the reason why we thought the army would have to garrisonned somewhere first, before being able to eventually, if the game was changed, be sent elsewhere. :)
Now you have to know this already exists in the game. :)
It's called coalition attack and can only be peformed towards collective targets (i.e. pantheons).
You first have to send your troops to the attack leader (usually the closer to the target), then he can lead an attack using up to 3 people's troops together to the target.
Now to try and finish studying every aspect of your question, I fear Plarium has already introduced a way to do what you're asking : city teleportation.
Now you can teleport close to each of your targets in turn, doing exactly what you asked for.
The problem is I personnally think it has more to do with Star Trek than ancient Sparta... :p