I always thought the technological difference was the weaponry used. If Zanarkand had the Aeons for military purposes, then they probably didn't think spending resources on machina weapons would be a good use. Bevelle, on the other hand, didn't have access to aeons or summoning magic, as far as I can tell, so they had to use a different technology, machina, to counteract Zanarkand's machina.

And Tidus's Zanarkand is pretty much the remembered fragments of the real Zanarkand, if only idealized, so they can all play their blitz in peace.

As for the summons, I'm guessing they could be used to protect the city for defensive purposes, or they could serve as 'guardians' to their summoners, who might have to be a certain class to have the summoning ability.