At the end of the day, it still comes down to this:
There are a few types of players
1) The noob - you can try to order them around, but as someone stated, they are still struggling with just the control of their demigod. Chances are, they will not even realize the chat - as i mentioned earlier, its quite easy to miss, esp. on some maps - and even if they do, unless you deliberately talk in a way thats free of TLAs and demigod terminology, then they might just not understand what you want. Not to mention, that quite a few of the 'leader' players are arrogant bastards and once the noob starts to hate the leader - and they hate arrogance, even if you are right - they will just refuse to listen. (Guess why a single polite word - please? - can work wonders compared to a wall of arrogant ranting)
2) The ignorant - these people are simply not team players. You can shout all you want, they will at most get pissed off at you and quit, or make you mad enough by their ignorance so you quit. Bossing them around is out of question.
3) The inexperienced team player - these are the kind who go help you when you do not need it, buy upgrades at the wrong time and generally try to be a good teammate, but they still have a few things to learn. Having a quick polite chat with them might help make them a better team player and giving a brief description of what tactics you think your team should follow might actually make them work rather well as a part of the team. OFC you can also just order them around and they will likely follow, as these type of players usually welcome authority and stick to the leader if there is some. Then again, attitude is very important with them as well. Generally speaking, arrogance never pays off on the long run. Always treat others, as you would like to have them treat you - that is.. with respect.
4) The experienced team player - If you are lucky enough to play with these, you rarely need to chat, as they will do the job of their demigod well and will also watch the overall game and go where they are needed. Most of the time communication is about - as someone already mentioned - leaving their post to go back to the citadel or crystal. Also pinging important locations works wonders with these kind of players, as it can draw their attention to important, low hp demigods, even though most of the time they will be observant enough to notice such battles without you pointing them out. Leadership in these type of teams is rarely needed, as there is usually an unspoken consensus on what to do. A few lines of tactical chatter usually come towards the end, or if the team is losing on how to turn it back, but even then its quick and rarely debated too much. (for example: i go caplock their corner flag while you hold the middle. upgrade to giants if you have the money)
Quite frankly, chat is not really enough for much more than what you have with player type 4. That is why voice comm - as usual - is such a huge advantage.