If Joseph hadn't stopped;
1: He'd look like he was in his 30s when he was in his 60s
2: Could have likely killed Dio by himself

actually it wouldn´t have helped him anything against DIO, Time Stop simply beats Hamon by a mile, he easily could have one shot him anytime, BUT it would have helped him later in Part 4 immensely due to being still in a pretty good state.