The problem is when the interview has absolutely nothing to do with the day to day work the hired employee is expected to do.
So someone who wants to be hired has to spend a lot of time and effort learning stuff that are only marginally useful - knowledge isn't bad per se, but it might be a bad allocation of resources.
I'm not saying that it is so in Google's case, only that I've seen and participated in interviews where questions had no relevance to the job description.
