When individuals have money in a joint account with a bank or financial institution, both have access to the funds. At the same time, any liability will fall to both parties.
For example, if the account is overdrawn, any fees associated with this will be deducted, regardless of who is responsible.
Even more importantly, debt collectors can usually seize the money in a joint account to satisfy one of the holders debts. This can happen in the case of:
In the event of death or divorce, the bank can prohibit access until clear ownership is determined, usually by the courts. Also, depending upon the state laws, the account might end up in probate.