Out of curiosity, why do they need pagers? Don't they carry radios on them?
I can't speak for my friends north of the border, but in general there are a few reasons for EMS personnel to carry pagers.
#1 Clarity of the call
You can get an exact address and info sent to a alpha pager so you do not have to ask the dispatcher for the address over and over (in case you are in a bad area or the dispatcher sounds like a drunk)
#2 Because they do have radios
Lets say everyone has a radio, if they have to leave it on all the time, then you will be using the battery power of the radio to just monitor all traffic (or just for your tones if you have a QCII enabled radio, but that still is a power draw).
#2.1 Not every person on an EMS crew might have a radio
#3 Extra/personal messages
In my county, they own their own paging network and they are able to also receive personal alphanumeric messages on their pagers. That includes all the volunteers for the local rescue squads as well as paid staff.
#4 EMS people just love hanging more crap on their belts, just ask them