Which rookie could take Widemans or Gio's ice timeSven?, Monahan?, Coborne? That is a product of lack of depth.
Doesn't have to be a rookie; simply playing the third defensive pairing more to see what they've got. Butler is after all, 'only' 27. And if Hartley mistrusts O'Brien so much, he might as well give someone from the farm or Smith a shot. He's leaning on guys who are giving him immediate returns - but that's not rebuilding, that competing for the present. developing players entails short term sacrifices for long term gains.
In any case, it's a well known strategy of Hartley's he plays his top guys a ton, while giving the bottom defense pair/fourth line very limited ice time. Bouma isn't going to do much or go anywhere with 4 minutes a game.
It's just interesting to see how other teams, ones who have actually been successful in developing skilled players over the last few decades, go about the process. Hertl for example, as a rookie on a cup contending team, already averages more time on the PP than either Baertschi or Backlund. And he plays on the Thornton's wing. The Sharks are nurturing his offensive game while knowing that the defense part can came in the future. The Flames seem to things the opposite way; demand flawless defense out of gifted offensive players, then lament the lack of offense. Maybe that's part of the reason all of the Flames attempts at developing a purely skilled offensive player (as opposed to physical offensive players like Iginla) had either failed miserably (Fata, Saprykin, Boyd, etc) or have had the players only find success after leaving the organization (St.Louis, Savard).