I don't have a problem with that (in fact, I would have loved to see Leivo make the team; maybe Broll, who I would easily take over Orr/FML). I don't know about Colborne's intensity (just because he doesn't hit everything that moves doesn't mean he lacks the passion to play), but I do know that, as you said, he didn't have a good camp. With that said, if a player who is physical and fights ALSO provides skill (the latter more important than the former, though physicality can be an aspect of that player's skillset), then I'm fine with that. If a player who fights/plays physical isn't talented, he is a waste of a roster space no matter if he's taking the spot from Colborne or Leivo or Broll.
This is another reason why the Grabovski buyout was short-sighted. The way you explain that depth, you are assuming that Bozak and Bolland (or McClement, for that matter) are average to above average centres for their roles. Bozak is most likely a 3rd line C on most teams, Bolland's play worries me in that he has been going down the last few seasons (and he played AWFULLY with Sharp and Kane last season, mind you in a short season), and McClement really blows at even strength (no matter where I look, I groan whenever I look at his 5v5 numbers; he's great on the PK, though). The only C you mention who should be mentioned in the top 6 is Kadri, which is fine but I wouldn't exactly call that "good depth".
Now, if someone goes down with an injury, the Leafs could sign a C like Steckel off the FA market if they have enough cap space.
I find that goons or designated fighters who can't play hockey to save their lives shouldn't play in the NHL, since they bring no value to a team whatsoever. People talk about how Orr/FML scares off teams from going after their star players, yet I see nothing of that sort on the ice. If you are physical/a fighter but can also pass the puck well, skate well, and/or shoot the puck at a good rate, I'll take you on my team.