Doc... on a normal team, even a normal good team like the Giants or the Astros - you are probably correct. You hide or ride the hot talent, and enjoy the benefits later.
Unfortunately this is the 2015 Mariners. We have Charlie, Biemel, and recently acquired Nuno as lefties in the pen currently. Plus, just last night the Rainiers used Dave Rollins for the first time and he was throwing 92 with movement, and a couple other pitches - and went over an inning without giving up a run in a meaningless game. Remember that Rollins is the Rule 5 guy that was suspended for 80 games... Mariners just played game 67, so that means Jack has 13 games to decide whether you keep him or you send him back. That's 4 lefties in the pen possibly right there on the July 4th. Plus Tyler Olson and Edgar Olmos in Tacoma just starting to pick up steam again in the Tacoma pen - so yes Montgomery is probably closer to 7th best lefty.
Jack has GOT to be making a deal shortly involving pitching again, or he is literally just throwing away talent. In that scenario, you don't add to your problem by putting Montgomery there too.
Thus, if this was a normal team with only a couple lefties again but...
It is probably the last homestand for a couple guys. May Jack choose wisely.