You are acting as if Montembeault has shown something in the past when that is not the case at all. He hasn't even played in a single playoff game, yet somehow he is supposed to be the "sure thing"?
If Montembeault was an experienced and successful goalie with a wealth of playoff experience you might have a point, but so far all he has shown is that he can be quite decent on a bad team, while having no experience handling the pressure of backstopping a good team (in general or in the playoffs). Under such circumstances, yes, it is much better for Toronto to run the two goalies who seem to be working well so far, and look to improve the team in areas that are not working as well.