There is weekend service on several RTM (formerly AMT), lines but it's limited - every two-three hours on the Deux-Montanges Line (a bit better on Saturdays than Sundays) and only 3-4 daily round trips on the Vaudreuil Line. On the Saint-Jérôme Line, it'd be like running the Barrie Line only as far south as Downsview Park and forcing a transfer to the subway (though I concede that it's an awkward route to Luicen-L'Allier). There are no train-buses. GO really spoils Lakeshore Line communities by comparison.
The STCUM used to be responsible for the commuter lines before the provincial agency took them over 1997, that might be one reason why they're shown on the STM maps.
Boston's subway maps show commuter lines as well, but they're controlled by the same agency.
The TTC's subway maps suffer from their poor dimensions. If the TTC adopted a portrait subway maps beside the doors with line-only maps above the doors (like New York), I could see the GO and UPX lines depicted on the subway network maps.