The game play says "Player X pulls up for an outside jumper, guarded closely by player Y". Question is, is it the same if all my other players have OD = 1 or OD = 12?
The player doing the close guarding (or otherwise specifically mentioned) is the only one whose rating matters on that play.
Now I understand what you mean about overall defensive ability, and that you have a good understanding of the team ratings.
As to the switches, I've noticed my team frequently scores when the other team switches and a guard tries to stop one of my big men, or one of their bigs jumps out to face my guards on a three. So I think of these switches as golden opportunities to score, or big stops when (usually) my team is not scored upon. So while a player is guarded by his counterpart most of the time, when he's not it is a key moment in the game, and ones you need to win as often as possible.