Agreed. Boxing is a huge detriment to the community of the server.
Not only does it promote isolationism but it also works to hinder creativity and skill. Players who can't box have to figure out ways to achieve their goals even when lacking a 'key class'. Group's tank has to go and there's no replacement? If you can't box, then you have to think outside the box (pun intended) and come up with a creative way to continue to earn XP without a tank. (Fear kiting, root rotting, etc). This makes everyone involved a better player and allows for some of those lesser used spells/items to get dusted off and see the light of day.
Boxing kills the community of any MMO. It's far to easy to simply settle in to always playing by oneself when you can box, or maybe having a two person group. Not only does it kill socialization, but it also means the world has to be bigger because if everyone is playing two characters, the content can't support as many players (there's only room for X groups in a zone... without boxing, that zone can accommodate X*6 players. With 2-boxing, it can only accommodate X*3, etc).
Don't let the lack of a 'key' class prevent you from achieving your goals. Simply come up with a different way to get there. Sure, it may be less efficient than following the holy-trinity of Tank/Healer/DPS, but some times you'll find it's MORE efficient than the normal methods (PoFire tables kite groups come to mind... they never wanted any melee!
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