I have a second account for my phone or machines that are not my own. Its more likely to get compromised, but its less valueable (not on DT and already has negative trust).
or do some fraudulent transactions and get fake trust ratings.
Maybe there are real reasons you would need more than one account, but I can't think of any right now...
I was inspired to do this by theymos and BadBear as both have similar "less valueable" accounts.
Another use would be privacy. If you have a well known main account and do not want your address to be known, create a new account.