Don't underrate the confidence and flow factors, either ... when you get the offence going, you want to keep that momentum going forward, for next week. I didn't see the end of the game - unfortunately all I saw were highlights - but the advantage of playing your starters late in a game that you're winning, ESPECIALLY after a major slump (2-game losing streak is a major slump in Montreal!), you want to instil confidence in your team, and try and get as close to a "well-oiled machine" as can be. Of course practice is good, but nothing compares to a game ... and if you get the first-string offence (for example) taking reps in a game, it's going to help a lot.
I do agree in taking out your starters to "save" them, but maybe if it's an easy game, against a weak team, and you've been on a roll for a couple of weeks. It's always gain vs loss - perhaps you save your starting QB, but perhaps your offence doesn't flow very well in the next game.
One last thing ... this is the CFL! Anything can happen in the last 5 minutes, even if a team is losing 32-9 I'd be nervous forcing my receivers to adjust to a new QB if there's not a huuuuuuuuuuuge lead, and with a questionable D (like Montreal's these days). Yeah, you could bring in your starter again, but then he's cold (better chance for injury) and the receivers have to adjust again ...
Just my 2 cents.