It's not that simple. The first REM line is actually great, it connects important hubs with populous suburbs and the airport. It makes sense to build it and it will probably improve integration and connectivity within the city. It also uses corridors that already existed. In many ways, planning for this was already done by the time the CDPQ came along (in the sense that the A10 corridor was already identified as a rapid transit corridor, the Deux-Montagne line was already deemed an important line for improvements, service to the airport was already being considered, the Mont-Royal tunnel has been a candidate for subway service since the 50s, etc). Even then, the government had to force the Caisse to add connections to McGill and Edouard-Montpetit metro stations.
The next phases of REM are another story. The government is giving la Caisse the mandate to plan for the next transit projects, and that's the scary part. They don't care about many parameters important to proper transit planning (and it's okay, their job is to get financial returns). Look at the Taschereau boulevard tramway project. The government simply took it off the hands of ARTM and the City of Longueuil to give it to the Caisse...
Having the Caisse invest in transit it deems profitable is fine, but they should not be in charge of planning. That's precisely why ARTM was created, and that's what they were in the process of doing until the government came in and took it all away, not even involving them in the REM de l'Est plan...