I have wondered the same thing before and asked myself “if I am defending properly, when would I have the time to type a message to someone else?”. So far I have not really made an answer to that question that makes enough sense for this chat to seem truly necessary.
Sure, if a base has an anchor, you have time prior to the dragon engaging towers to coordinate first few supershots. After that you should be dropping hammers so fast that typing is not possible. Seems more feasible to have a general defense strategy worked out in your team so you know what to prioritise and cycle. You can’t stack supershot loads so if you go to hit the Ice Flak and can’t cast, you know the other teammate has and you can load another tower or wait to cycle the next IF supershot. Defenders are active parties and all active parties in the defense cannot actively chat.
Only one person can fly at a time so those who aren’t are passive participants at that time; therefore, they have more time to send small notes to the flier making them have a more active role. I don’t think this is a large advantage as you cannot see loaded supershots earlier and the skill of the flier is still the critical element to pulling off the flight.
As for seeing both chats. I’m not for this for the same reason we cannot see eachother’s team chats. This is a strategy game and a team’s comm lines should remain restricted to them to preserve that element of strategy.