It helps in the scenario where someone comes in on a Thursday or Friday and says 'i need a bat for the weekend' - I think most people assume that most bats come knocked in and ready to go nowadays. It's quite a shock when they say/ask this and i have to correct them, thus limiting their last minute choice to GM.
It can also help with people who don't want to knock in a bat themselves or pay us to do it.
I can remember someone turning up to a game straight from JJB sports one week, wielding a brand new GM Hero.
When someone asked why they'd chosen that one their reply was "I bought it half an hour before the meet for the game, this was the only one that didn't need 4 hours worth of knocking in while I drove here" as good a reason as any I guess!