Dunn: in principle, I'm in favour of drafting the best player available... but he is too similar to Rozier.
Unless Danny thinks he is an outstanding prospect, not much point in getting a third PG and risking a Reggie-Jackson-in OKC situation.
I wouldn't worry too much about a Reggie-in-OKC situation.
Is Rozier an NBA rotation player? Hard to say. Seems likely he's a CJ Watson-esque combo guard at best, which means you don't necessarily want him as the primary ball-handler.
Is Smart a primary ballhandler? Hard to say. I think he's a bigger version of Patrick Beverley. At best on offense he's Eric Gordon.
With all of that in mind, Dunn looks like much more of a pure point guard prospect. If he's the best guy available at #6, he probably becomes the backup ballhandler out of the gate, with Evan Turner moving on in free agency.
Drafting a backup ballhandler at #6 could actually make sense if that guy has above-average-starter upside (e.g. Reggie Jackson). By the time Dunn is finishing his rookie contract, Isaiah Thomas will be on the wrong side of 30.
So, draft Dunn, let Turner go in free agency, and we have a guard rotation of Thomas / Bradley / Smart / Dunn next year, with Rozier and Hunter as depth in case of a trade or injuries.
In a couple years, Bradley might move on if he's not traded already (since Smart and Rozier duplicate his role), and Dunn could move into the long term starter role once Isaiah begins to decline.
All of that said, I'd much prefer for the Celts to get a really nice prospect at center or on the wing, but if Dunn is clearly the best guy at #5 / #6, I think the Celts should take him.