Sometimes you just need to respond to something that was written now.. either because the other guy might be leaving or maybe someone reacted wrong and you can't wait because then you would have to explain WHEN he did WHAT wrong
Most of the time when people chat they either like to complain about teambalance or try to help anyone in some way so i generally think people that are CLEARLY chatting at the moment should not be fragged if possible!
Sometime its however hard to quickly see someone is chatting. In huge maps it can happen that you see the enemy color and shoot an rocket without any chance to see the chatsign. Well thats bad luck then..
Chatting in the enemy base however is really really a bad spot and i can understand if people frag there anyway. My personal point of view there is: If i think i can defend my base even with that guy around he might live.. that means Mirio: i would frag YOU if you were in my base as you are too good ^_^