Actually, this is NOT in stark contrast to what Doc did during the 'Big 3' era. Doc typically made a point of keeping Ray, Rajon or Paul on the floor with the bench as much as possible. That's one of the reasons Ray was typically our total-minutes leader in those first few years (followed closely by Rondo).
Also, this has been an evolving point for Stevens. The critical runs made by both Dallas and Toronto (to stretch a lead from 8 points to 19 points in the former, and to get back into the game in the latter) were both made when he essentially subbed out all of the starters. An examination of the game logs shows this to be the case very clearly. In each of those cases, the subs started with Green & Sully going to the bench first, but then the other starters going to the bench soon after. And each time the game got out of hand quickly.
And again, during the OKC game, it was upon the substitutions to bring in the bench when the game turned south.
The most killer moments have been during subs at the end of the first period and then also near the end of the 3rd period.
Right now, I think there is a huge drop-off in performance between our starters and maybe one or two others (Bass & Zeller) and the rest of our roster. And this really shows in the NetRatings (plus/minus) numbers for the roster. Those two and the 5 starters are either positive or only mildly negative for the season. The rest of our bench are mostly deeply negative (except for Young, but his minutes have been tiny). Turner, Pressey, Wallace and yes, even Smart, all have deeply negative ratings so far.
So it makes sense for Stevens to try to move to a more gradual substitution pattern that overlaps the starters with the bench, but ultimately, we need better performances out of our bench.