Let's try this a different way, Favors has no use to Boston long term because they would have to trade better players to avoid the luxury tax in order to keep him. Most teams trading for Bradley would be doing so in order to acquire his Bird Rights and lock him up long term.
As for your silly claims on the defense, what kills him is that he's 6'2" playing most of his minutes with a 5'9" running mate, meaning that he has to do all the heavy lifting. The teams that would have interest in Bradley aren't running their offense through an undersized PG. So Bradley would be in the position of doing what he does best, defending PGs and shooting open threes.
Again, we disagree. Or rather, you're making assumptions I'm not willing to make. We don't really know what the Celtics roster will look like next summer, which is part of why I like Favors for them in the short-term. Favors allows them to address a need without having to use a big asset or commit to any longterm money.
As for whether or not Favors makes sense longterm in Boston, I'm not sure anybody is really in a position to say. The idea that they would necessarily have to trade better players to avoid the luxury tax is far from a certainty, and if that is indeed the case, they don't have to keep him and all they've lost in the process is Avery Bradley who the C's will also have a lot of difficultly keeping. Favors allows them to address a position of need and free up minutes for Fultz, Rozier, Smart, etc. It's hard to project where this team's cap is in a year. Do they bring back Olynyk? At what price? Amir? What's IT worth to them? What do they do in free agency this year?
As for my "silly claims on defense," you just provided a reason why my silly claims are correct. He hasn't been a good defender because he has no positional versatility. He can guard one position, and can only be matched with another guard who can run the point on defense but defend 2s. That's a limiting factor for any player, and when you throw in the fact that he's an upcoming UFA, it depresses his trade value even further. The board has had this conversation -- maybe even in this thread -- and it's pretty hard to find an obvious trade fit for Bradley. Houston gets mentioned a lot, but I'm not sure why Houston would prefer paying Bradley more when they have Beverley and Gordon who, combined, make less than Bradley will in FA. Philly was linked to Bradley at the deadline, but apparently wasn't interested in Boston's price. Given where they are as a franchise, their first round picks are going to be off the table, and Holmes should be too. A cheap, cost-controlled PF with upside should be worth more to Philly than Bradley.