Cant you see this is the problem. If there is tons of other 8 man groups half of the team will be 8 man groups, and a pug will and a 5 man might lose to that every single time. This would be a catasrophy.
The casual pvpers wouldnt be able to get a single win. And what fun is there for a 8 man group to meet a team that you will run over in 2 minutes and noone in your team dies. I dont really see the fun in that..