A pre-made team's greatest asset is coordination. That is born of voice communication (in many cases) as well as practice. Coordinated and effective teams function as a cohesive unit. This is their greatest asset and is something that a randomly assembled team of PuGs can never hope to emulate. Eight strangers drawn from different time zones, locales, and classes that have no prior knowledge of each other or their group's composition can never operate as a seamless unit in a warzone at a moment's notice. Not even with added voice communication.
That is why PuGs and pre-mades should be separate. No randomly assembled team can ever spontaneously match the skill and coordination of an existing pre-made. No way. No how. No where.
With the exception of voice chat coordination, any 8 players who know how to play can and will work perfectly together. Even if all 8 are not, 3-4 working together can achieve much the same effect. I offer an example of -just me- using an understanding of group play and strategy to affect the outcome of a match.
Civil War. The enemy team has captured mid and west, and the pug's on my team (I'm pugging it this round) are struggling to take a node. After wiping on the west turret, several of us go mid on respawn. I head back west as the enemy is running back towards mid. There are 2 guards on the turret, and 4 of them coming to defend mid. Now a average or bad pug at this point might have tried to ninja cap with 2 guards, but someone understanding group play launches into the group for heading toward mid, blowing stuns, mezzes, and roots. I delayed them about 20 seconds, and we captured mid.
With the exception of voice chat (which is a seperate issue I've mentioned before), the -only- difference between a good 4 man premade or 4 good pugs is a premade was grouped before the match.
Or as someone else put it, Premades don't make 4 good players, 4 good players make premades.