The down side of this is that someone accepted then will focus on the startup instead of finishing up their studies. You could argue that would happen either way but I think 'having it in the can' means more chance of doing so. And perhaps also not uncovering and turning down other opportunities or overlooking them. (Similar to when you buy a house you are not going to any other open houses..ditto for buying a car and many other things).

How are you supposed to focus on learning and school work if you already known that you are getting in YC to do a startup? Which more often than not is going to fail instead of focusing on your studies which you are paying big dollars for (potentially at least).

Also to a smaller degree you are giving up info to YC which in theory would also allow them to identify a trend to fund someone else ahead of you. (Relevant if they turn you down).