The lottery system makes it very difficult to effectively tank your season in the NBA. Basically no matter how bad you are, there is always a chance that you will not get one of the top 3 picks (in fact, the worst team in the league has rarely been awarded the top draft pick).
Here are the odds that the worst team in the NBA gets a pick at or better than the current pick, starting with pick 1. (assuming 2 teams do not tie for the worst record)
- First pick: 25%
- Second pick: 46.5%
- Third pick: 64.3%
- Fourth Pick: 100% (if the worst team in the league does not get one of the top 3 picks, they are automatically awarded the 4th pick).
Now, the fourth pick is still a pretty good player, and there is a pretty good chance that you are going to get the second or third pick. However, depending on the draft it may not be the kind of no-brainer that the top pick often is. Therefore tanking in the NBA is only a good strategy if you believe the draft is good enough that the top 3-4 players in the draft are potential franchise defining players.
The question of "has any team ever purposely tanked" is an incredibly difficult one. Plenty of teams give up and trade major pieces at the trade deadline in both basketball and baseball. However, no player would admit to playing less than 100%, and coaches are often fired if their team performs poorly, regardless of the players they have on the team, so it's not in their best interest. So while management can, and will put teams in positions to fail, the players and coaches usually try to do everything they can to not be that last place team.
A great example of this is interviews with Indianapolis Colts players this past year, they knew they were bad, and would be assured a new franchise QB if they had the worst record, but they also knew their jobs were at stake if they didn't at least try their hardest. And at the end of the year when they were the worst team in the league. Despite the fact that they were in position to draft a brand new franchise quarter back, they fired their GM. The players, coaches and management reiterated time and again that "there is no suck for Luck going on.
The other thing that goes into the equation of whether or not to tank is the question of what do you owe the folks that pay the bills. Namely your fans and your television and radio contracts. If you trade away your best players, is the product that you put out on the floor for the last 30-40 games of the season worth the (often obscene) ticket cost your fans are paying? If they stop showing up because you are awful this year, will they re-up their season tickets next year? Even if you only get the 4th pick of the draft? Will your TV and Radio partners give you a poorer contract when it comes time to re-up if they know that you have a tendency to throw a season away for potential draft picks?
In summary, it's an incredibly hard decision for management and ownership when it comes to the trade deadline about whether or not to trade away your best players and play for picks. But (at least on the public face) it's always a clear choice for players and coaches, play as hard as you can and try to win the games that you can. Unless as a coach you have a clear direction from management to sit your best players in the final couple of games, you will be trying to win what games you can so that you might actually get to keep your job at the end of the year. And as a player you're interest is in making sure that you put yourself in the best position to continue playing your sport, which means playing as well as you possible can.