I'll be frank and honest - I think people who refuse to withdraw their other choice when they have an offer from their first choice without adequate reason (i.e. KNOWING one offers AAA and the other A*AB and you don't think you might get AAA) - are just being plain selfish. I believe some of them just want to be able to tell others in real life, or tell the applicants for the next year on TSR, that "Oh I got 4 offers for medicine so my advice is more important and I'm ****ing fantastic" so they can say "Wow Sherlock, you're amazing. I'll listen to you". Well there..that's my opinion - they're being selfish.