While I am someone who wants the Celtics to make getting a top pick their #1 priority, I also think they can do it w/o tanking. You can play young guys a lot, and work on becoming much better at specific plays & fundamentals all while not having a heavy focus on winning games.
The issue that I have is with the logic that getting a top pick is not the fastest and/or most reliable way to build a championship. It very clearly has been the best route in the league, with the rare exception of 1 or 2 desired locations for free agents (i.e Lakers).
The Celtics need maximum value this year. That value will come through obtaining a top draft pick, the development & showcasing of current young talent (i.e. Bradley, etc), and expiring contracts. Getting a bunch of mediocre free agents and fighting for the 8th seed will not yield the value that this team needs to assemble a championship caliber squad.