I'ma concur with icefractal's final paragraph - it looks like there might be a discrepancy between the kind of game you want to run and the kind of game your players want to play.

It sounds as if they want to play a game where they need not attend to their defences. Meanwhile, you want to run a game where character builds with such vulnerabilities are unwise.

There's nothing wrong with either their or your preferences, but they do seem to present a significant compatibility problem.