To summarize:
Over the course of the series, the Suns are better off WITH Johnson than without.
In Game 1, Jim Jackson stepped up nicely.
It is very common in the NBA, when a player is out, to have somebody else step up and fill that role in a particular game. What you can't count on is having that role filled night in and night out.
It is folly to think that Johnson would make no difference for Phoenix.
I think it also folly to think that Johnson would represent a significant upgrade over the performance Jackson gave in Game 1. Suns fans should not count on that. What they should count on is the inevitable dropoff in production at the 2 that would happen eventually if Johnson continues to sit. Given that the Spurs won Game 1 even with Jackson playing so well, the Suns would be hard-pressed to win the series in that case. They need Johnson back.
What Johnson provides that was missing is not so much shooting the ball from the perimeter, since Jackson did that just fine, as in backing up Steve Nash at the point MUCH better than Barbosa, and in having at least a prayer of keeping Tony Parker away from the rim.
Joe Johnson is not the X-factor that would swing the series back in Phoenix's favor, but as short as their bench is, they need him back to compete effectively.