Let's say someone creates a ticket in our system. We may need to be contact 3th parties for the ticket. We perhaps would contact someone else (his boss, a supplier) through mail. In GLPI you have to do that through other means, and log the result in GLPI once done.
The problem with this is that it's much harder to have teams working on a ticket, as some info is handled outside the ticket.
GLPI could allow the agent to pick a user to send a mail to. (This mail could be marked as private even by adding something to the mail, that way a reply would be private too).
This is something that is handled for example in OTRS in a very good way.
Customer support service by UserEcho