Back to the discussion on the actual value of a 3-3 stone, it's also interesting to note that the value in points of a stone can change throughout the course of the game.
A classic example is the value of a 4-4 stone:
$$W
$$ ------------------
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . X . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
- Click Here To Show Diagram Code
[go]$$W
$$ ------------------
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . X . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .[/go]
Of course, white can play at the 3-3 point, and take away the corner territory that black has. For example:
$$W
$$ ------------------
$$ | . . . . . . . . .
$$ | . . . 9 0 . . . .
$$ | . . 1 2 . . . . .
$$ | . . 3 X . . . . .
$$ | . 5 4 . . . . . .
$$ | . 7 6 . . . . . .
$$ | . . 8 . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
- Click Here To Show Diagram Code
[go]$$W
$$ ------------------
$$ | . . . . . . . . .
$$ | . . . 9 0 . . . .
$$ | . . 1 2 . . . . .
$$ | . . 3 X . . . . .
$$ | . 5 4 . . . . . .
$$ | . 7 6 . . . . . .
$$ | . . 8 . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .[/go]
$$Wm11
$$ ------------------
$$ | . . . . . . . . .
$$ | . . 1 O X . . . .
$$ | . . O X . 2 . . .
$$ | . . O X . . . . .
$$ | . O X . . . . . .
$$ | . O X . . . . . .
$$ | . . X . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
- Click Here To Show Diagram Code
[go]$$Wm11
$$ ------------------
$$ | . . . . . . . . .
$$ | . . 1 O X . . . .
$$ | . . O X . 2 . . .
$$ | . . O X . . . . .
$$ | . O X . . . . . .
$$ | . O X . . . . . .
$$ | . . X . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .[/go]
Black has no points in the corner, but the 4-4 is still considered to be an acceptable play, because even with an invasion of the 3-3, black has outward influence, and will likely get points in other areas of the board. And in fact, early in the game, this is probably better for black than white. White gets a few points, but black has potential extending toward the center of the board.
Given this, we might estimate some value for the play at the 4-4 point. You could say, given the territory black can make eventually from the resulting influence of a 3-3 invasion - combined with the possibility of enclosing this corner, etc... We might have some value X for the amount of points we estimate for the 4-4.
But as the game progresses, this changes. Imagine the same 4-4 point, but there are other white stones that are strong, surrounding:
$$W
$$ ------------------
$$ | . . . . . . . O .
$$ | . . . . . . . O .
$$ | . . . . . . . O .
$$ | . . . X . . . O .
$$ | . . . . . . . O .
$$ | . . . . . O O . .
$$ | . . . . . . . . .
$$ | . . . . . O . . .
$$ | O O O O O . . . .
- Click Here To Show Diagram Code
[go]$$W
$$ ------------------
$$ | . . . . . . . O .
$$ | . . . . . . . O .
$$ | . . . . . . . O .
$$ | . . . X . . . O .
$$ | . . . . . . . O .
$$ | . . . . . O O . .
$$ | . . . . . . . . .
$$ | . . . . . O . . .
$$ | O O O O O . . . .[/go]
Now, if white invades, assuming same play by black:
$$W
$$ ------------------
$$ | . . . . . . . O .
$$ | . . O O X . . O .
$$ | . . O X . X . O .
$$ | . . O X . . . O .
$$ | . O X . . . . O .
$$ | . O X . . O O . .
$$ | . . X . . . . . .
$$ | . . . . . O . . .
$$ | O O O O O . . . .
- Click Here To Show Diagram Code
[go]$$W
$$ ------------------
$$ | . . . . . . . O .
$$ | . . O O X . . O .
$$ | . . O X . X . O .
$$ | . . O X . . . O .
$$ | . O X . . . . O .
$$ | . O X . . O O . .
$$ | . . X . . . . . .
$$ | . . . . . O . . .
$$ | O O O O O . . . .[/go]
Black could even be killed. So in this type of situation, the value of the 4-4 has changed.
This is an extreme example, but the point is, as the game changes in the surrounding board, the value of stones change. What is possible early on in the game starts to become impossible later on as moves are played.
Because of this, it's difficult to give a static value to a group of stones, because it depends highly on the placement of stones in other areas of the board.