Don't get me wrong; the three-point shot is vital to this team's success, and it has been ever since we've had Duncan, Manu, and Ginobili getting the ball into the paint and then dishing out to shooters. As you mentioned, it also spreads the floor for the post players, and causes defenses to scramble and break down with good ball movement. Last but not least, it's the great equalizer. How many games have we seen the Spurs bring back, or pull away in, with a three-point barrage this season?
However, the problem with the three-point shot is, and always has been, that it is streaky. Even teams that are amazingly consistent with the three are capable of putting up stinkers like that 3-17 night against Philly. Sometimes the shots just don't go in, for whatever reason.
Suddenly, all of those misses not only become empty possessions, but they also become long rebounds that put more pressure on the transition defense. The longer you continue to clang away from outside, the more crowded the paint gets. Ultimately, if the Spurs don't perform from the arc, then it makes the game very difficult to win.
These games are easy to dismiss in the regular season, but if your team is reliant on the three-pointer in the playoffs, one or two off nights can be the difference between a ring and an early vacation.
It's a necessary bet that the Spurs (and really, all teams) are obliged to take, but it's still a gamble.