Think about the possibility of massive attacks on locations. Saya group of clans/regiments come together and invade a part of let's say Venus. I'm not sure how many people can be in a single area but if there is controllable ship travel then events like this would be awsome.
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Well, in order for this to work, Bungie would have to design the space for it, and I really don't see much call for dozens of Guardians coming together at an appointed time to do a single thing together. Maybe they could have really big public events, and you could get everyone into a public space and wait for it to happen, but other than that I don't really see the point. Also, controllable ship travel is probably not a thing.