I think you need to consider that the plan A for Marcus Smart is to become the perfect version of Avery Bradley that Danny always wanted.
He's freakishly similar to Dywane Wade physically and skill wise, and Dywane Wade fit perfectly with Jason Williams when the Heat won the championship in 2006.
If Rondo and Avery and can work, then Smart and Rondo can work.
My guess is that Danny's two plans are:
Plan A :
Rondo starts at PG.
Smart starts at SG and works on his shooting.
Try and get Smart up to something like 35% from three and a more reliable jumpshot- but the focus is to get Smart the ball in the paint and around the basket so he can get to the free throw line..ala Dywane Wade.
When Rondo rests, let Smart play PG and see how he does at the NBA level. Remember he's only played strict point guard for one season, in his second year of college.
Rondo is perfect for finding a guy like Smart slashing and cutting to the basket, just like he did with Avery Bradley, he'll be perfect for finding Smart and Young coming off screens and getting them the ball in the right place for the best shot.
The reason this back court can possibly work out well is because they are both aggressive bulls who want to win. Smart, like Rondo, can guard most NBA shooting guards because of his wingspan and strength.
Plan B:
Smart gets given a season to see if he can play shooting guard and replace Avery Bradley long term at SG. If he looks terrible at SG, but he looks good at PG then it's more incentive to not over pay Rondo in free agency and let Smart rule the roost.
I think that Danny and Brad Stevens like the idea of putting Smart in a disciplined motion offense system so he is forced to take the shots that the offense gives him- the same way that the Pacers force Lance Stephenson to go through their offense.
They let him go wild sometimes when they need a spark, but particularly in his first year or two he'll need some structure and Rondo is the PERFECT guy to make sure he understands how an offense works at both PG, while Smart learns SG.
Rondo is only ball dominant in the sense that he facilitates the offense- because no one else can. Pierce could run it well, but Avery was hopeless. He's never had a decent backup point guard in all his time here other than Dooling and Eddie House. He's not like Tony Parker who's had Ginobli, Patty Mills, George Hill, Bruce Bowen etc. who can all run an offense very well.
Even more importantly, he has only run Doc's offense with good players- and he did it very well. Stevens will encourage a multiple guard set up like Butler or San Antonio and Rondo has two nice pieces in Smart and Young to work with on the perimeter.
You've got arguably the best passing point guard in the world- Rondo, and a beast of a combo guard in Smart who will attack the basket and work on his shot. They'll probably try and make it work, and Stevens must think it can possibly work because he's had a huge say in these picks working with Rondo.