All valid points. The only thing I'm thinking about is if the counter reset after each turn, it may become more difficult for proper refrencing of prior events. Example: Say I'm a mason. It's turn 4. I have to deal with c04.003, m04.010 and whatever else is thrown out. After awhile, the "prefix" letter will be lost to the eyes. By keeping a running tally of all messages sent, it just seems easier (IMO).
Just to clarify, I'm not against the letter prefix, just the message count reset each turn.