I agree with you that in most situations, trying to find a young guy with "fringe rotation role player" upside from the D-league makes more sense than signing guys over 30 who have name recognition but not too much else. Still, from a win-now perspective, it's unlikely that most guys from the D-league are going to be as valuable as a Keyon Dooling, Leandro Barbosa, Mickael Pietrus sort. But I definitely think it makes more sense than bringing in guys like Mikki Moore, Troy Murphy, Stephon Marbury, Shelden Williams, etc. The point is just that Greg Stiemsma is the exception to the rule.
Now, as for really "filling out the roster," I think once the team is truly rebuilding, that's when it makes sense to sign a good number (e.g. 3-4) of D-leaguers and fringe-NBA players -- your Dionte Christmas, Kris Joseph, Stephen Lasme, Craig Brackins types. That increases your chances of finding one or two who might actually develop into role players moving forward. A lot of decent players have come out of the D-league, but they usually get their shot at significant minutes on a rebuilding team first.
The added bonus is that when you're rebuilding, you don't care about winning. In fact, winning is kind of a bad thing. So you let your draft picks and young assets play a ton of minutes in inflated roles, and then you fill out the "supporting cast" with guys who mostly aren't good enough to contribute to a winning NBA team. Anybody who sticks is found money.
Ideally, the Celtics would be like the Spurs. They'd go dumpster diving and come out with some gold and jewels. Somehow the Spurs take guys like Danny Green -- cut from the Cavs -- and find a solid starting SG. Or they sign an undrafted free agent and get a productive backup combo guard like Gary Neal. Or they draft an overseas guy nobody has ever heard of with a second round pick, let him develop for a few years on somebody else's dime, and it turns into Tiago Splitter or Luis Scola, or at least Pablo Prigioni or Ime Udoka. The Rockets are pretty good at this too. I'd say those are the two best teams in the league for player development in the bottom half of their roster.
That's another thing -- once they are rebuilding, I'd like to see the Celtics stockpile second round picks and try to find some role-players-in-the-rough there, as well. Seems like every year, a few guys who were really productive in college fall into the second round. Those guys have a tendency to become decent NBA players. They just don't have the home run upside, so bad teams stay away from them for some reason. Except teams go from being bad to good by stockpiling legitimate players.