BPA - always. If the Rangers felt Montoya was the best guy at that spot, the only thing they do by selecting another player is to give the team drafting after them a gift.
I'm sure the Rangers explored their options to trade down, but they didn't like what they heard. So they took the goalie, and figured if they ended up with a surplus of quality goalies, they could always trade one away later.
Besides, does everyone realize the fail rate of even top ten picks? The odds that Blackburn, Lundqvist, Montoya, and Labarberra all make it is infinitesimal. The odds that three of them make it is miniscule. The odds that two of them make it? Still probably less than 50%. And that has nothing to do with the Rangers scouting. That's just those are just the historical probabilities.
In the end, if the Rangers get one top quaility young goalie, they made out like bandits. How many draft picks would you give, top ten or otherwise, to have a Luongo, Turco, or Theodore on your team?