Here’s a nice analysis of playoff series, in the NY Times.
In 2003, the league changed the opening round of the playoffs from best-of-five to best-of-seven, matching the final three rounds. Of the 80 first-round series since, the team that would have won a best-of-five series has won 76 times. All told, there has been a lot of unnecessary basketball played, with a scant few instances where a lengthier series has changed the outcome.
Only four teams — the 2010 Atlanta Hawks, the 2007 Utah Jazz, the 2006 Phoenix Suns, and 2003 Orlando Magic — have come back to win a series in which their opponent was the first to three wins. There have been 13 win-or-go-home Game 7s, but nine of them have been won by the team that would have won a best-of-five series.
The longer series hypothetically protects the top seeds from being eliminated in three fluky losses, but no scenario has arisen where a No. 1 or No. 2 seed has come back after losing three games.
The best-of-seven is ideal for later rounds, which usually pit evenly matched elites, and the result of each series carries more weight. But first-round pairings tend to be lopsided, and there have been more sweeps — 16 if you include the Heat and Spurs this year — than Game 7s. In the same time span, the latter playoff rounds have produced only seven sweeps total compared to 13 Game 7s.
Read more here: