Typically, in a European city, I would question why a project like the REM was being built. Just use lightweight EMU's that run on traditional rail.
But here, we aren't allowed to do that. Without huge red tape and tons of exceptions from Transport Canada. Its a logistical nightmare.
So, I really think that this is the best option, using this LRT style system for regional rail, simply because the trains we would be allowed to use for mainline rail would cause the cost of any elevated transit to balloon in price, and you would be very limited to how the trackage could be weaved between other infrastructure. In fact I dont even think the new Champlain bridge can support heavy rail.
So, this is the best option, in Montreal/Canada.