Cap space. Cap space. Cap space.
The end.
He doesn't cost any cap space since he is a free agent. Which is why I just don't buy that argument especially given his price.
Depending on who he was traded for of course he would take up cap space.
If he was traded for expiring contracts his cap hold would have eaten into the max cap space available.
If he was traded for someone who was still going to be under contract for next year, depending on how much salary went out, a part of his $10.9 million cap hold would eat into the cap space available for a max free agent
But you don't have to have a cap hold. You can just let him go if you need the space to sign someone else. So he takes up no cap space. In fact if he was traded for say Rozier, you would have actually created cap space by letting him go because you wouldn't have Rozier's contract on the books.
Given how shaky Cleveland looked at the deadline (which has continued) and with Durant injured, this was a prime time for Boston to make that sort of minor move, which might have actually you know helped the team this year. This hanging onto what might be someday stuff doesn't work all that well when you had a top 2 team in the conference at the deadline. Make the moves, improve the team, and try to win the championship this year, especially when you aren't giving up much of anything to do it. Look if Noel had cost a ton, I get not wanting to give up real assets for a potential rental, but Noel went for a mid tier prospect and some 2nd rounders. Boston has plenty of those and isn't going to be able to fit them all on the team in the future anyway.