For a bat to perform at it's best you want a thin hard layer over a soft lower layer of willow, creating a trampoline type effect. Knocking in too much makes that layer too deep, meaning that performance is decreased.
Laminated bats create the hard layer artificially (though do still need some preperation!), and is one of the reasons why they perform.
That said, as a manufacturer, you want people to knock in a bat as much as possible as it means less returns.