I'm not sure what you mean by "def rating" but this is a game of plus/minus.
It's all about what you value most. If I'm an AL team, I don't mind using a defensive SS and C because I have 7 other spots to get my runs. Unless, of course, I don't have a hitter at one of those 7. If you have 3 non-hitters in your line-up, you're essentially giving up 3 innings per game.
In the NL, you've got the pitcher's spot and any other non-hitters. Since I value SS defense over any other positional D, my catchers in the NL tend to be offensive players.