A lean deck can be very good by the end of the game, but the problem is getting to that point. On the harder difficulties it's often better to focus on choosing

A lean deck can be very good by the end of the game, but the problem is getting to that point. On the harder difficulties it's often better to focus on choosing strong cards that help you now rather than worrying too much much about thinness. So it's a good strategy still, it's just that most of the time you can improve your deck faster by adding good cards than by removing bad ones.