Simultaneous turns is the holy grail for almost any turn-based empire management game. There are plenty or reasons - like the mentioned unpredictableness, added strategy elements and more careful planning among others.
But ultimately there is one aspect which makes this vastly superior to IGOUGO in these type of games:
Wait times between turns would be almost non-existant
compared to current system, because the calculations are done (unless the coders are idiots) while the player is actually playing the game.
There could be a problem if they have intentions to shove their ugly graphics down our throats by modelling every rabbit and tree and







and sphincter and fireworks going off at the same time and looping their animations endlessly when "end turn" button is pressed. That could eat memory and processing power.
But there are dozens of ways to counter it - not making stuff graphic intensive, teleporting units, using info screens and turn summaries, priority systems where movement is shown if it's enemies or near your cities or it's battle for example.
Essentially in normal circumstances this system would mean that when player presses "end turn", next turn begins almost immediately because the AI players have done their turns already, leaving waiting time only for calculations for city growths, production, movement and such.