The process can definitely reject good candidates who are out of practice, but it’d be pretty hard to do the practice and give good performances without some underlying skill.
But look at it that way: a candidate that didn't practice before the interview, is either not so interested, a bit dull about basic human interaction or cannot even understand what an interview is.
It's honestly like coming to an interview without a suit and a tie. Sure if you're a pretentious startup it'll horrify you, but anywhere else it shows that the candidate just doesn't understand what's happening, that he could have just made the cheapest first impression effort and didn't even think about it, he came in sandals and tshirt instead.
Do you want him talking to clients ? Can you afford him not talking to clients ?
