I see it as pointless and a waste of everyone’s energy. You’re insulting the student and making him/her emotionally let go of the school–premature if you do actually accept him/her later. There’s nothing like feeling wanted and it’s hard to shake feeling unwanted.
It’s like someone asked you to the prom and you answered “I probably won’t go with you.” Just freaking say no.
Better to just send an outright rejection letter IMO, the earlier the better.