Interestingly, the range in supporting cast quality appears to be quite enormous. This jibes with the range of NBA teams that we see in simple SRS observations, scaling from about 15 points worse than opponents to 13 points better.
Supporting casts get better as teams get better -- teams who miss the playoffs have the worse supporting casts, whereas teams who reach the Final 4 have the best supporting casts. The average supporting cast in the NBA (2001-2012) is -5.3/48 minutes. Here's the breakdown:
And in graphical form. Note that finalists have the smallest range in variation between Supporting Casts while teams that reach the PS but are eliminated the 1st round have the largest range in variation:
Without their best players, the worst teams in the NBA have been as bad as -19/48 minutes. This is right in line with what we'd expect based on SRS, as the team would have to only improve to -13/48 with their star (not much of improvement) to finish as a -14.5 SRS team. On the flip side, the best teams in the league have supporting casts that are over +5.0/48 without the best player. This represents a different of about 25 points -- in other words, the difference between the best and worst supporting cast is greater than the impact of the best individual player we've ever observed since +/- has been tracked.
"Best player" was defined as the player on a team with over 1500 MP in a season who produced the best net on/off per 48 minutes.
"Supporting Cast" was then defined as the performance of the team per/48 when that player was off the court.
EDIT: There is a typo in the chart -- the mean Finals cast is -2.5