I don't think the REM is the answer to all of Montreal's transit woes. Sometimes a metro extension will make more sense, sometimes commuter rail, sometimes better bus service. I also don't like the idea of suburban REM stations being designed as nothing more than giant park and ride lots. I expect something like that for commuter rail, but the REM should be a medium density option where a good chunk of people walk/bike, a sizable portion take buses there and a minority drive. To be honest, I think it would make sense to convert the Metro yellow line to an REM. This should make it easier to extend further, as Longueuil is around the perfect density for the REM.
I think that to qualify for a station there should be a few requirements: e.g. a minimum number of people within walking distance and sidewalks along all the main roads to the station (e.g. maybe 5,000 within 800m for all stations along the line until it drops below that density).
There's an interesting look at the density around the Metro/Commuter rail stations in Greater Montreal here:
http://www.cat-bus.com/category/map/
In the case of east end Montreal it makes sense to build some sort of transit extension out there. For the density and distance to cover in that part of the island, maybe an REM would make sense, although I would suggest that should be done through a conversion of the Mascouche line, with another branch being built south, like in the West Island.
The other proposed REM extensions are just vote buying and will lead to more sprawl. Brossard south of the 30, Chambly or northern Laval are nowhere near dense enough to merit something like the REM. They they would be better served by commuter rail (to gather people from a wider area) or express bus service to commuter rail/REM stations.
Right now, there's all sorts of brand new manufactured sprawl being built south of Autoroute 30 in Brossard in what was previously farmlands. That money would have been better spent densifying already built areas closer to downtown that are lower density such as Saint Hubert. Easier said than done for sure.