It's an apples and oranges thing: this is of Comiket, so everyone involved has different motivations from shoppers desperate to grab the doorbusters. You'd have to compare to another convention or event.
Their FAQ mentions that if you come on the first train, you'll have to wait for 5 hours, but if you come at the opening time of 10:00, you only have to wait approximately one hour. The fact that people are willing to spend those extra 4 hours to get in early seems to indicate that it provides an advantage to be one of the first.
Incidentally, they expressly prohibit waiting in line overnight. I bet plenty of people would do so if they could.
They still do. Every year its a big issue and the organization committee makes a press release. The night before the most recent Comiket there were reports of several thousand overnighters. The 徹夜組 (overnight group) cause so much trouble and garbage that it endangers Comiket's hosting arraignment, or so is claimed.