If the C's signed Horford, I would be much more comfortable sticking him at the 5 and drafting a PF like Bender/Chriss than Poetl. Horford is already in his 30s, and with the PF position kinda disappearing, Bender/Chriss at least provide more flexibility in the frontcourt.
Personally, I'd rather have Poeltl then Horford. Horford isn't looking too impressive right now, and passing of time won't make him any better.
While I generally agree, I think the veteran presence is important. I do consider Horford or even P Gasol to be an upgrade over Sullinger or Amir for the short term. Any rookie will take time and guidance to develop as we all know. A veteran just makes that a smoother transition IMO.