Interesting question. This was the sensible route for Montreal - Quebec City from the outset, since for the longest time VIA's QC service terminated south of the river without actually crossing back to Gare de Palais. However, at the time "CP" trains didn't terminate at Montreal Central either, VIA had several other long-distance trains also using the CN tracks on the south shore and AFAIK there is still no connector between the commuter tracks north of Gare Central to the CP line. Currently all VIA services out of Montreal - even the Northern Quebec routes - use the one, sometimes more circuitous, routing to get where they're going.