More often than not, an aircraft crossing an active runway would be on ground freq. Are you saying that for the sake of situational awareness all YYZ traffic should be on one frequency?
I've never been to Montreal tower, so can't comment on procedures there, but in the UK, whenever an aircraft is to cross an active runway it is always transferred to the frequency of the Air Controller for that runway. Even if it is just for that brief journey from the holding point one side of the runway to vacating the opposite side. It is then transferred to the appropriate Ground Controller.
Having all traffic on one frequency would be nonsensical at a busy airport such as CYYZ. However, having aircraft due to cross, and hence potentially infringe an active runway, it does make sense to be on the same frequency.