If the Celtics clear cap space they can make a run at free agents next off season. I think the likes of Kevin Love (if he isn't dealt), LaMarcus Aldridge, Marc Gasol, Big Al (if he opts out) etc would rather have Rondo at PG than Smart...
I think Rondo is a bit of a unique player though. He is a great defender, rebounder, and passer, but he does not score enough to a true number one player. I think his value really comes into play with him as a second or third guy that does so much.
He makes a lot more sense on a playoff team, so I think the Celtics probably move him. If you can get a decent return for him, it makes more sense than hoping that the Celtics will be championship caliber in the next 3-4 years.
If the Celtics can add a top tier free agent and hit lucky in the draft, they can start building themselves into a competitive outfit. Granted it might make sense to deal Rondo and get something for him, but what will a team offer for him?
The Kings might throw a first round pick their way plus Maccllum and McLemore; or a contender might throw some picks their way; but whatever the deal, unless it was too good to turn down, I'd keep Rondo and re-sign him.
I think Rondo, Bradley, Green, Young, Smart and co makes for a decent young core. Add another set of draft picks and add a big name free agent, and they can make progress. Some teams have risen quickly, so it wouldn't surprise me to see the Celtics back as a force in the next couple of seasons.