There is the legal risk (it's illegal). Also, its a bad practice for, well, a lot of reasons. You assume that all balances are positive, and that they haven't

There is the legal risk (it's illegal). Also, its a bad practice for, well, a lot of reasons. You assume that all balances are positive, and that they haven't used that account for auditing purposes, or to commit some plain old investor fraud by claiming to have more cash than they do. What happens if the balances aren't recorded accurately? There's a million reasons that commingling funds is incredibly bad practice.