The case in point is that if that, to quote a simple example, if the opponent plays an outside tactic with a great offensive player at PG, adding defense at any position other than SG will not be very useful -- regardless of what the weight of that position's defense is in the game ratings.
Game simulation is based on individual matchups. Any weights you assign to individual positions in the calculation of the aggregate team ratings are arbitrary, because they don't take into account the opposing team.
The team ratings are not used in the game but somting similar is. So individual matchup is one part, but there is one that should be quite close to the team ratings as well.
In my eyes that part tells you how well the team screens,cuts, dumps etc. Its also like if the only good scorer is the point guard the rest of your defenders can sag off and help much more on the pg. This is simulated with something that looks like the ream ratings.