No brainer IMO.
Skilled big men are a LOT rarer in this league than skilled PG's. We'd have a far better chance of picking up an all-star PG in a draft then we'd have of picking up an all-star big man.
We already have Bradley, Lee, Terry and Crawford in the backcourt which is a very solid guard rotation. In the front court though we are paper thin.
If we made this change I see two scenario. Near future I see Hortford and KG in the front court with Sully coming off the bench - that's an exceptional frontcourt. Distant future I see Sully and Hortford in the front court with (whowever we have by that time) coming off the bench.
Hortford is a fairly young and very talented big man who can pass, defend, rebound and also score in a variety of ways. He's a legit all-star calibre big man who is genuinely capable of playing the center position effectively (and spent plenty of time doing so in Atl).
Future lineup of AB, Lee, Green, Sully, Hortford? Plus $25M in cap space to add more guys (KG / Pierce / Terry will be gone by then)?
A lot of potential there...
Also I dont get why people are saying Hortford is not a good enough return for Rondo. Hortford is an All-Star caibre big man in a world where All-Star calibre big men are rare and overvalued. He plays on both ends of the floor, he's strong yet athletic, he's a good rebounder and passer and (unlike Rondo) he is not a borderline head case.
The Celtics will NEED a high quality frontcourt guy to take over once KG leaves, and neither Sully nor Melo is likely to be that guy - both are more likely to become very good support players at best. They already have a young and talented backcourt for the future, so if you have a chance to pick up a young and talented big man you MUST take that deal.