It looks too much like a 1 and 3 mushed together, even when people write relatively neatly. If they have sloppy handwriting it leads to inevitable tragedy.
This wasn't such a big issue for me in America, where one could usually discern letters/numerals through context. But here in the UK where postcodes are a mix of numbers and letters it's all too confusing.
I'd like to send an e-petition to Downing Street to be made the government's top priority for Things Next To Be Ignored, but as I'm not a citizen yet I can't even do that. I'll have to sit here being ignored unofficially and attempting to decipher people's Bs, 1s, and 3s.