Only guy it would make sense to trade for Speights is Bass.
Speights isn't an upgrade on Bass, so I don't think it makes sense from Boston's perspective.
Not sure many would agree with you on this one...
And those people are wrong. Or at least, they are wrong in the short term, as I expect most new players to need 2-4 months to get comfortable on defense. I think it's a lateral move, that wouldn't move the team any closer to a title because you still have a huge drop-off behind the top two bigs on the team. If Speights had a couple of years left on his contract, I might feel differently. If Speights is going to decline his player option and move on, then it is a horrible trade for both teams.
Speights is a guy who I think could make this team a contender next season if the Celtics had the full MLE and used it on him. A big man rotation of KG, Bass, a healthy Sullinger, and Speights with occasional cameos by a Jason Collins type fifth big who shows up when the team needs a big body would be a perfectly acceptable situation for the team next season.