Min, I reapplied to Caius after getting rejected for Englsih, without pooling, by Selwyn.
Imo, the best reason to reapply is if you feel that *not* reapplying will sour your experience of whichever uni you do get into. I was eventually rejected by all my choices except Warwick and Southampton, and I really did not want to go to either. Therefore, I reapplied!
It's easy to say it was a good decision now, as I've got my place at Caius - but I do remember feeling a huge emptying of anxiety when I finally made the decision, so it was clearly the right personal choice at the time, despite the risks involved.
If you're worried about staying sharp, may I suggest taking an Open University module in your subject to cover the 'gap year' time between your A2 exams and the Oxbridge interview?
I started an Open University course of about the equivalent of 2nd year uni study. There was a £250 grant on offer (as I earn little to no money of my own) and it was free (for the same reason), and it helped me stay on the ball.
I hope you make whatever choice is best for you, I really do. And if you make it with more than three months to spare, you'll be well ahead of me.
Good luck!