Unless they start its pretty bad form. If it is used it can only be for social stuff or for there info eg organising a first solo Never for an instruction or a request for information.
Sometimes it seems to be used to bypass the student when they want to talk to the instructor and the student is on the RT eg xxxxxxx i have an inbound in 10 mins how many T&G's have you got to go.
Also the ATCO is human as well and works with the instructor day in day out and its not unknown for another freq to be given for box 2 to get that nights beer drinking plans sorted out.
MJ