The first question one has to ask is how effective that player is when he is on the field? A player who is extremely effective, but only on the field 40% of the time is better than an ineffective player that is on the field 100% of the time (cough...Carlos Rodgers...cough). If this is true, than why is the effective player taken off the field? Because by being effective in a game, offenses adjust and change their schemes away from that player. Look at the following scenario:
1st down -- Offense is in a basic alighnment, Niner defense is in their taditional 3-4 with a NG. Offense rushes the ball. Because of an effective NG anchoring the center of the rush defense and eating up blockers so Willis and Bowman can run free to the RB, the offense only gains 1 yard.
2nd down 9 yards to go -- Offenses usually will change to a more spread formation by substituting the FB with a slot WR. Niners defense responds by substituting the NG with another DB and move into their hybrid defense. Lets say the offense passes short and gains a few yards.
3rd down 5 yards to go -- Offenses again will spread out more usually substituting another WR for the TE. Defense responds but does not add a NG. Lets say the offense does not pick up the 1st down and punts.
So for this series, the NG was on the field for only 33% of the defensive plays. However, the contribution given on the 1st down was key in limiting what plays the offense would run in their attempt to get the 1st down. Unfortunately for the NG, being good got you put on the bench for 2nd & 3rd downs.
Is there another scenario that had an effect on this NG playing percentage? Would the fact that the Niners had a good lead in games affect things? Sure, if the opponent is playing catch up (like the New England Patriots game) and passing almost every down, then the Niners are not going to be in NG-needed formations. This drops the percentage as well.
I am not going to go into the whole can Dorsey play NG at his weight or is Williams experienced enough because that is open to each person's opinion. I was trying to address the usage of this stat as justification for the Niners not to draft a NG.
BTW: First Fanpost on Ninersnations. Critiques will be respected.