Adventureland and Frontierland are not open for EE, so many off-site guests rope drop those at regular opening time. As others have said, the lines (especially for non-headliners) are still pretty short in the 9-10 am hour. If you happen to be coming on a party day, there is a huge advantage to rope dropping at 8 am since most guests will not get to the parks at 7:30 am for EE. (But those days will mean no fireworks for you without a party ticket.)