When you go through the Clockworks in the Arcade, you start out, either by yourself, or with a buddy. Soon, people start joining the party. Some leave the party, and other people take their place as THEY join the party. You all fight together, in clockwork tunnels, danger rooms, arenas, graveyards, ruins, maybe even boss levels, exploring under the surface of Cradle.
Then, when you get to the Core or a Town (depending on the tier), the TRUE treasure is neither the crowns you get, nor the minerals, materials nor the heat. The TRUE treasure is cooperation, teamwork... FRIENDSHIP. Not just adding the party members to your friends list, but working with each other, fighting together, and even though you don't know you they REALLY are, even though their identity is hidden behind their player names, you still remember them, almost like true friends in real life.
Friendship is worth, like, a derpbillion crowns, and those who can cherish it make the most of their adventuring. (even if they used a lot of energy reviving! :D)
-Pery
I think this is the biggest thing people lose out on when they only do missions. You can't join missions midway through, and there's a limit to how long you can be bothered waiting around in a lobby. But arcade parties last for a good while and you pretty much never go through one alone, despite how much doomsaying people are making about the place.
Go run the arcade, dudes and dudettes! Meet some buds, have a good time! A lot of Spiral Knights have changed and will continue to change, but the Arcade is still there if you stop fixating on direct crown output (which isn't even that bad if you go all the way through).