As far as I can tell, Going to Orbit is a pointless load screen for eye candy's sake. Why not instead of Go To Orbit, you hold the button to bring up the exact same menu? Skip a load screen (of which there are a large number of, half of which being Going To Orbit) in the process?
Edit: To those saying it's a lobby and for fireteams, I'm not saying get rid of the functionality of Go To Orbit, I'm just saying lose the load screen and backdrop, and just have it a menu you can access straight away by using the same Press and Hold that we use now. No one's instanced 'physically' that you invite while you're in Orbit now, so there's no worry about what inviting people to your Fireteam and Party might be like. It would change nothing
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My theory on the Orbit system is that like others are saying it is a "lobby". This is necessary because it gives them 1 server that can handle all the players connecting to the game. When you select a destination and we see the loading screen, I am assuming that the 1 server we are on sends us to a machine called a Load Balancer. The Load Balancer picks a server running your selected destination and has the lowest load and/or best network connection. This helps with the complete instancing of all areas, and helps them keep lag to a minimum. When the load times are taking longer I assume it is the Load Balancer taking a bit longer to find a proper server. Another thing I think they are able to do because of this setup, is slowly update/patch the game without taking all the servers down. Because of this Load Balancer set up they can shut off 1 server sending all players to orbit. The Load Balancer wont see the server anymore so wont try to send any player to it. Bungie can then update the server while everyone plays on the other ones, then turn the server back on and rinse and repeat till all the servers are up and running. To address the OP's edit: No one is "instanced" physically period as that would be playing on LAN. The issue with having Fireteams and Parties join you directly is that it would by pass the server selection process. Having one or two groups do this wouldn't cause to much of an issue, but if everyone did this it would increase the server load significantly and would cause more lag.