Two users are not allowed to have the same e-mail address. I would say that 99% of the time, that is just fine.
However, say someone attempts to register a new account and has a problem. For example:
viewtopic.php?p=51545#51545
Their next step would probably be to try to register again. If they have only one e-mail address, this doesn't work because it's a duplicate and they are essentially locked out of the boards.
Not only can they not get in, they can't even post for help.
That was just one example of how an account might be messed up, but this problem applies to anything that would bugger an account. Password change gone bad, e-mail change gone bad, etc..