Raccoon Jesus
Todd McLellan is an inside agent
You know, that's a great way to look at it.
Use Rielly as an example since the OP mentioned the Leafs. When used as the #1 guy to go against top lines, the other teams are at the advantage, not the Leafs. Whereas if Rielly is leading the 2nd pairing as the top guy, he excels in that role instead of trying to just hang on.
Kind of how I feel as well. Guys like Reilly, Muzzin, Hamilton, etc. are fantastic players in their own right but if they have to consistently face top competition or play over 20-22ish all-situation minutes a night, their play starts to suffer. as you go deeper in the playoffs vs. stronger opponents and controlled matchups, that's when these guys gets exposed unless they're on a hot streak--that consistency is what separates them from the 'true' #1 types. I'd feel wholly comfortable (and frankly you'd have one of the best d-cores in the league) with any of these guys as a #2 complement or #3 guy leading the pairing, but though they can function as a #1 in bursts, long-term, they're not ideal.