It doesn't matter if it's a light-hearted RP event or a high-intensity progression run. As sure as the sun rises, if everyone is told the event is going to begin at 5 PM sharp, at least one player will not show up until 5:01. They might claim timezone difficulties, traffic, or even just that they forgot what time it was taking place. Inevitably, someone shows up late, and quite frequently several people arrive late to a lower-impact event. That's not counting the people who show up on time, but have one last thing to finish that takes upwards of half an hour.
Of course, MMOs are fundamentally hobbies, and it's understandable that not everyone will be perfectly punctual. But by the same token, people who delay an event's start time by a significant amount are cutting into the enjoyment of other players, and it does mean they're harder to rely upon. How long do you wait for late arrivals before starting without them? Will you kick people from a regular roster, or argue that they should be kicked, in the event that they fall behind schedule too frequently? Or are you on the other side of the equation, and despite your best efforts you're almost always late to the party?