Wasteful would be not giving the interview every ounce of effort that we had. I have a passionate hate for the Caltrain and didn't want any additional reason to stress on the morning of the YC interview. Showing up to the interview frustrated or stressed seemed more wasteful.
I am trying to understand why I was downvoted for this.
Have nothing to do with your downvoting but the author explains why - 'No need to stress out more than we already were.'. Whether or not they expensed a hotel room could strike many as tangential to what is most interesting about being interviewed by YC.