You can accept an valid signed transaction with low risk even if it hasn't confirmed yet because in a blockchain w/o regular massive backlogs the chances that it doesn't confirm are very low. So for smaller every day transactions there isn't a need to wait for an actual confirmation.
In a situation as exists with Bitcoin currently it's anyone's guess whether a transaction will ever confirm because there are HUGE backlogs that haven't cleared for several months. A transaction has a high chance of being dropped regardless of the fee being paid because the backlog and fee level required to get confirmed at all is unpredictable. Bitcoin Core also has what they call RBF which increases the chance that a dishonest person can double spend an unconfirmed transaction exponentially. Makes 0-conf transactions unusable on Bitcoin Core.
