I disagree, at least in general.
First, even though the Celtics have to spend up to the salary floor, that doesn't mean they have to take on bad long-term contracts for role players. Rather, they can sign guys to one year deals, which is a much better route.
Second, trading for good-but-not-great guys on long-term deals is the way to guarantee mediocrity going forward. I would rather see the team maintain its cap space from year to year, rebuilding via the draft.
Now, if we're talking about trading for guys with another year on their contract in return for a draft pick, that makes a lot of sense, I think, especially when looking at our free agency options this summer. If somebody wants to throw us a pick for absorbing one year of bad salary, fantastic. Two years, it becomes more iffy, because at that point we're losing some free agent opportunities and are probably delaying our rebuilding. Three or more years I don't think makes a lot of sense at all.