Joining up for the chance to play and run into other players without having to fear having hours of work wiped out. I work nights, and as it is don't get as much game time in as I want, so I don't much care for the idea of getting taken out coming back from a long exploration expedition, or after loading up a newly acquired ship for a trading run.
There are certainly times I'll think about popping into open play to experience some PVP and/or get involved with groups like the Hutton Truckers and their events, but I'd prefer to be able to pick and choose when I might risk getting blown out of the sky by another player
