Well, I may be a little conservative, but airlines donīt like it and may take some action if you "miss" the last segment regularly (some airlines even have a clause in their T&C that they have the right to recalculate the fare) and the fare difference usually isnīt very high for different departure cities within Canada, so IMO itīs not worth it. (Also donīt forget that you only have a contract to be flown from CDG to YUL, so if there are any mechanical/weather issues you may be rerouted on a nonstop flight to YUL)

If I were to use this strategy, I would use it on routes ex-US via Canada (e.g. SEA-YVR-LHR) or ex-EU to NA where the fare difference is several hundred $$.