What would be amazing is if you could place pins on the map inbetween attacks so players know where to go. I hate trying to tell someone, go to the NW corner right where the two lines meet next to the flame thrower.... Let's put a PIN in it!
Having the ability to message in an operation isn't a bad idea because it would imbalance the game. It's a bad idea because people need to focus when they are attacking. Say someone is having some trouble knowing what to do while the battle is going on. More than likely trying to read and send messages is only going to deter them from keeping an eye on the battlefield. Tips need to be done during planning outside of the fight, preferably in a place where task force members can give visual aid of what needs to be taken out before the next person goes in.
In retrospect, if the person attacking looks like they have no idea what they are doing, more than likely they disregarded the note, kept silent, and attacked without changing their troops to something worthy of use. Getting their attention is would be hard enough to begin with if they are trying to fight.